diff --git a/codex/rng.nim b/codex/rng.nim
index 19452cd4..c16fef56 100644
--- a/codex/rng.nim
+++ b/codex/rng.nim
@@ -15,7 +15,7 @@ import pkg/libp2p/crypto/crypto
 import pkg/bearssl/rand
 
 type
-  RngSampleError = object of CatchableError
+  RngSampleError* = object of CatchableError
   Rng* = ref HmacDrbgContext
 
 var rng {.threadvar.}: Rng
diff --git a/codex/sales.nim b/codex/sales.nim
index 01f3da77..9b50b9e5 100644
--- a/codex/sales.nim
+++ b/codex/sales.nim
@@ -4,7 +4,6 @@ import pkg/upraises
 import pkg/stint
 import pkg/chronicles
 import pkg/datastore
-import ./rng
 import ./market
 import ./clock
 import ./proving
@@ -13,7 +12,7 @@ import ./contracts/requests
 import ./sales/salescontext
 import ./sales/salesagent
 import ./sales/statemachine
-import ./sales/states/downloading
+import ./sales/states/preparing
 import ./sales/states/unknown
 
 ## Sales holds a list of available storage that it may sell.
@@ -74,10 +73,7 @@ func new*(_: type Sales,
     reservations: Reservations.new(repo)
   ))
 
-proc randomSlotIndex(numSlots: uint64): UInt256 =
-  let rng = Rng.instance
-  let slotIndex = rng.rand(numSlots - 1)
-  return slotIndex.u256
+
 
 proc handleRequest(sales: Sales,
                    requestId: RequestId,
@@ -87,17 +83,15 @@ proc handleRequest(sales: Sales,
     slots = ask.slots, slotSize = ask.slotSize, duration = ask.duration,
     reward = ask.reward, maxSlotLoss = ask.maxSlotLoss
 
-  # TODO: check if random slot is actually available (not already filled)
-  let slotIndex = randomSlotIndex(ask.slots)
   let agent = newSalesAgent(
     sales.context,
     requestId,
-    slotIndex,
+    none UInt256,
     none StorageRequest
   )
   agent.context.onIgnored = proc {.gcsafe, upraises:[].} =
                               sales.agents.keepItIf(it != agent)
-  agent.start(SaleDownloading())
+  agent.start(SalePreparing())
   sales.agents.add agent
 
 proc load*(sales: Sales) {.async.} =
@@ -110,7 +104,7 @@ proc load*(sales: Sales) {.async.} =
       let agent = newSalesAgent(
         sales.context,
         slot.request.id,
-        slot.slotIndex,
+        some slot.slotIndex,
         some slot.request)
       agent.start(SaleUnknown())
       sales.agents.add agent
diff --git a/codex/sales/salesagent.nim b/codex/sales/salesagent.nim
index 372c3da8..81858702 100644
--- a/codex/sales/salesagent.nim
+++ b/codex/sales/salesagent.nim
@@ -1,8 +1,13 @@
+import std/sequtils
 import pkg/chronos
 import pkg/chronicles
+import pkg/questionable
+import pkg/questionable/results
 import pkg/stint
 import ../contracts/requests
 import ../utils/asyncspawn
+import ../rng
+import ../errors
 import ./statemachine
 import ./salescontext
 import ./salesdata
@@ -13,10 +18,13 @@ export reservations
 logScope:
   topics = "sales statemachine"
 
-type SalesAgent* = ref object of Machine
-  context*: SalesContext
-  data*: SalesData
-  subscribed: bool
+type
+  SalesAgent* = ref object of Machine
+    context*: SalesContext
+    data*: SalesData
+    subscribed: bool
+  SalesAgentError = object of CodexError
+  AllSlotsFilledError* = object of SalesAgentError
 
 func `==`*(a, b: SalesAgent): bool =
   a.data.requestId == b.data.requestId and
@@ -24,7 +32,7 @@ func `==`*(a, b: SalesAgent): bool =
 
 proc newSalesAgent*(context: SalesContext,
                     requestId: RequestId,
-                    slotIndex: UInt256,
+                    slotIndex: ?UInt256,
                     request: ?StorageRequest): SalesAgent =
   SalesAgent(
     context: context,
@@ -39,6 +47,43 @@ proc retrieveRequest*(agent: SalesAgent) {.async.} =
   if data.request.isNone:
     data.request = await market.getRequest(data.requestId)
 
+proc nextRandom(sample: openArray[uint64]): uint64 =
+  let rng = Rng.instance
+  return rng.sample(sample)
+
+proc assignRandomSlotIndex*(agent: SalesAgent,
+                            numSlots: uint64): Future[?!void] {.async.} =
+  let market = agent.context.market
+  let data = agent.data
+
+  if numSlots == 0:
+    agent.data.slotIndex = none UInt256
+    let error = newException(ValueError, "numSlots must be greater than zero")
+    return failure(error)
+
+  var idx: UInt256
+  var sample = toSeq(0'u64..<numSlots)
+
+  while true:
+    if sample.len == 0:
+      agent.data.slotIndex = none UInt256
+      let error = newException(AllSlotsFilledError, "all slots have been filled")
+      return failure(error)
+
+    without rndIdx =? nextRandom(sample).catch, err:
+      agent.data.slotIndex = none UInt256
+      return failure(err)
+    sample.keepItIf(it != rndIdx)
+
+    idx = rndIdx.u256
+    let slotId = slotId(data.requestId, idx)
+    let state = await market.slotState(slotId)
+    if state == SlotState.Free:
+      break
+
+  agent.data.slotIndex = some idx
+  return success()
+
 proc subscribeCancellation(agent: SalesAgent) {.async.} =
   let data = agent.data
   let market = agent.context.market
@@ -78,13 +123,16 @@ proc subscribeSlotFilled(agent: SalesAgent) {.async.} =
   let data = agent.data
   let market = agent.context.market
 
+  without slotIndex =? data.slotIndex:
+    raiseAssert("no slot index assigned")
+
   proc onSlotFilled(requestId: RequestId, slotIndex: UInt256) =
     asyncSpawn data.slotFilled.unsubscribe(), ignore = CatchableError
-    agent.schedule(slotFilledEvent(requestId, data.slotIndex))
+    agent.schedule(slotFilledEvent(requestId, slotIndex))
 
   data.slotFilled =
     await market.subscribeSlotFilled(data.requestId,
-                                     data.slotIndex,
+                                     slotIndex,
                                      onSlotFilled)
 
 proc subscribe*(agent: SalesAgent) {.async.} =
diff --git a/codex/sales/salesdata.nim b/codex/sales/salesdata.nim
index d8226877..3f12fc79 100644
--- a/codex/sales/salesdata.nim
+++ b/codex/sales/salesdata.nim
@@ -8,7 +8,7 @@ type
     requestId*: RequestId
     ask*: StorageAsk
     request*: ?StorageRequest
-    slotIndex*: UInt256
+    slotIndex*: ?UInt256
     failed*: market.Subscription
     fulfilled*: market.Subscription
     slotFilled*: market.Subscription
diff --git a/codex/sales/states/downloading.nim b/codex/sales/states/downloading.nim
index b2b57958..da7b95f1 100644
--- a/codex/sales/states/downloading.nim
+++ b/codex/sales/states/downloading.nim
@@ -9,12 +9,12 @@ import ./errorhandling
 import ./cancelled
 import ./failed
 import ./filled
-import ./ignored
 import ./proving
 import ./errored
 
 type
   SaleDownloading* = ref object of ErrorHandlingState
+    availability*: Availability
 
 logScope:
     topics = "sales downloading"
@@ -36,9 +36,7 @@ method run*(state: SaleDownloading, machine: Machine): Future[?State] {.async.}
   let data = agent.data
   let context = agent.context
   let reservations = context.reservations
-
-  await agent.retrieveRequest()
-  await agent.subscribe()
+  let availability = state.availability
 
   without onStore =? context.onStore:
     raiseAssert "onStore callback not set"
@@ -46,18 +44,8 @@ method run*(state: SaleDownloading, machine: Machine): Future[?State] {.async.}
   without request =? data.request:
     raiseAssert "no sale request"
 
-  without availability =? await reservations.find(
-      request.ask.slotSize,
-      request.ask.duration,
-      request.ask.pricePerSlot,
-      request.ask.collateral,
-      used = false):
-    info "no availability found for request, ignoring",
-      slotSize = request.ask.slotSize,
-      duration = request.ask.duration,
-      pricePerSlot = request.ask.pricePerSlot,
-      used = false
-    return some State(SaleIgnored())
+  without slotIndex =? data.slotIndex:
+    raiseAssert("no slot index assigned")
 
   # mark availability as used so that it is not matched to other requests
   if markUsedErr =? (await reservations.markUsed(availability.id)).errorOption:
@@ -83,7 +71,7 @@ method run*(state: SaleDownloading, machine: Machine): Future[?State] {.async.}
 
   trace "Starting download"
   if err =? (await onStore(request,
-                           data.slotIndex,
+                           slotIndex,
                            onBatch)).errorOption:
 
     markUnused(availability.id)
diff --git a/codex/sales/states/filled.nim b/codex/sales/states/filled.nim
index c5318cb2..ff3f0ffc 100644
--- a/codex/sales/states/filled.nim
+++ b/codex/sales/states/filled.nim
@@ -23,7 +23,10 @@ method run*(state: SaleFilled, machine: Machine): Future[?State] {.async.} =
   let data = SalesAgent(machine).data
   let market = SalesAgent(machine).context.market
 
-  let host = await market.getHost(data.requestId, data.slotIndex)
+  without slotIndex =? data.slotIndex:
+    raiseAssert("no slot index assigned")
+
+  let host = await market.getHost(data.requestId, slotIndex)
   let me = await market.getSigner()
   if host == me.some:
     return some State(SaleFinished())
diff --git a/codex/sales/states/filling.nim b/codex/sales/states/filling.nim
index 251b97e7..4d775da2 100644
--- a/codex/sales/states/filling.nim
+++ b/codex/sales/states/filling.nim
@@ -28,4 +28,7 @@ method run(state: SaleFilling, machine: Machine): Future[?State] {.async.} =
   without (collateral =? data.request.?ask.?collateral):
     raiseAssert "Request not set"
 
-  await market.fillSlot(data.requestId, data.slotIndex, state.proof, collateral)
+  without slotIndex =? data.slotIndex:
+    raiseAssert("no slot index assigned")
+
+  await market.fillSlot(data.requestId, slotIndex, state.proof, collateral)
diff --git a/codex/sales/states/preparing.nim b/codex/sales/states/preparing.nim
new file mode 100644
index 00000000..51f43733
--- /dev/null
+++ b/codex/sales/states/preparing.nim
@@ -0,0 +1,67 @@
+import pkg/chronicles
+import pkg/questionable
+import pkg/questionable/results
+import ../../market
+import ../salesagent
+import ../statemachine
+import ./errorhandling
+import ./cancelled
+import ./failed
+import ./filled
+import ./ignored
+import ./downloading
+import ./errored
+
+type
+  SalePreparing* = ref object of ErrorHandlingState
+
+logScope:
+    topics = "sales preparing"
+
+method `$`*(state: SalePreparing): string = "SaleDownloading"
+
+method onCancelled*(state: SalePreparing, request: StorageRequest): ?State =
+  return some State(SaleCancelled())
+
+method onFailed*(state: SalePreparing, request: StorageRequest): ?State =
+  return some State(SaleFailed())
+
+method onSlotFilled*(state: SalePreparing, requestId: RequestId,
+                     slotIndex: UInt256): ?State =
+  return some State(SaleFilled())
+
+method run*(state: SalePreparing, machine: Machine): Future[?State] {.async.} =
+  let agent = SalesAgent(machine)
+  let data = agent.data
+  let context = agent.context
+  let reservations = context.reservations
+
+  await agent.retrieveRequest()
+
+  without slots =? agent.data.request.?ask.?slots:
+    raiseAssert "missing request slots"
+
+  if err =? (await agent.assignRandomSlotIndex(slots)).errorOption:
+    if err of AllSlotsFilledError:
+      return some State(SaleIgnored())
+    return some State(SaleErrored(error: err))
+
+  await agent.subscribe()
+
+  without request =? data.request:
+    raiseAssert "no sale request"
+
+  without availability =? await reservations.find(
+      request.ask.slotSize,
+      request.ask.duration,
+      request.ask.pricePerSlot,
+      request.ask.collateral,
+      used = false):
+    info "no availability found for request, ignoring",
+      slotSize = request.ask.slotSize,
+      duration = request.ask.duration,
+      pricePerSlot = request.ask.pricePerSlot,
+      used = false
+    return some State(SaleIgnored())
+
+  return some State(SaleDownloading(availability: availability))
diff --git a/codex/sales/states/proving.nim b/codex/sales/states/proving.nim
index c7694c5b..e9342eec 100644
--- a/codex/sales/states/proving.nim
+++ b/codex/sales/states/proving.nim
@@ -31,5 +31,8 @@ method run*(state: SaleProving, machine: Machine): Future[?State] {.async.} =
   without onProve =? context.proving.onProve:
     raiseAssert "onProve callback not set"
 
-  let proof = await onProve(Slot(request: request, slotIndex: data.slotIndex))
+  without slotIndex =? data.slotIndex:
+    raiseAssert("no slot index assigned")
+
+  let proof = await onProve(Slot(request: request, slotIndex: slotIndex))
   return some State(SaleFilling(proof: proof))
diff --git a/codex/sales/states/unknown.nim b/codex/sales/states/unknown.nim
index 6cf4164e..d586c6b7 100644
--- a/codex/sales/states/unknown.nim
+++ b/codex/sales/states/unknown.nim
@@ -27,7 +27,10 @@ method run*(state: SaleUnknown, machine: Machine): Future[?State] {.async.} =
   await agent.retrieveRequest()
   await agent.subscribe()
 
-  let slotId = slotId(data.requestId, data.slotIndex)
+  without slotIndex =? data.slotIndex:
+    raiseAssert("no slot index assigned")
+
+  let slotId = slotId(data.requestId, slotIndex)
 
   without slotState =? await market.slotState(slotId):
     let error = newException(SaleUnknownError, "cannot retrieve slot state")
diff --git a/tests/codex/sales/states/testfilled.nim b/tests/codex/sales/states/testfilled.nim
index 3e943d1b..89239bd1 100644
--- a/tests/codex/sales/states/testfilled.nim
+++ b/tests/codex/sales/states/testfilled.nim
@@ -28,7 +28,7 @@ suite "sales state 'filled'":
     let context = SalesContext(market: market)
     agent = newSalesAgent(context,
                           request.id,
-                          slotIndex,
+                          some slotIndex,
                           StorageRequest.none)
     state = SaleFilled.new()
 
diff --git a/tests/codex/sales/states/testpreparing.nim b/tests/codex/sales/states/testpreparing.nim
new file mode 100644
index 00000000..6f5d8c7f
--- /dev/null
+++ b/tests/codex/sales/states/testpreparing.nim
@@ -0,0 +1,29 @@
+import std/unittest
+import pkg/questionable
+import pkg/codex/contracts/requests
+import pkg/codex/sales/states/downloading
+import pkg/codex/sales/states/cancelled
+import pkg/codex/sales/states/failed
+import pkg/codex/sales/states/filled
+import ../../examples
+
+suite "sales state 'preparing'":
+
+  let request = StorageRequest.example
+  let slotIndex = (request.ask.slots div 2).u256
+  var state: SalePreparing
+
+  setup:
+    state = SalePreparing.new()
+
+  test "switches to cancelled state when request expires":
+    let next = state.onCancelled(request)
+    check !next of SaleCancelled
+
+  test "switches to failed state when request fails":
+    let next = state.onFailed(request)
+    check !next of SaleFailed
+
+  test "switches to filled state when slot is filled":
+    let next = state.onSlotFilled(request.id, slotIndex)
+    check !next of SaleFilled
diff --git a/tests/codex/sales/states/testunknown.nim b/tests/codex/sales/states/testunknown.nim
index d5e714e8..3faf4bb5 100644
--- a/tests/codex/sales/states/testunknown.nim
+++ b/tests/codex/sales/states/testunknown.nim
@@ -26,7 +26,7 @@ suite "sales state 'unknown'":
     let context = SalesContext(market: market)
     agent = newSalesAgent(context,
                           request.id,
-                          slotIndex,
+                          some slotIndex,
                           StorageRequest.none)
     state = SaleUnknown.new()
 
diff --git a/tests/codex/sales/testsalesagent.nim b/tests/codex/sales/testsalesagent.nim
index 6c621efd..fcfe2ea4 100644
--- a/tests/codex/sales/testsalesagent.nim
+++ b/tests/codex/sales/testsalesagent.nim
@@ -1,6 +1,3 @@
-import std/sets
-import std/sequtils
-import std/sugar
 import std/times
 import pkg/asynctest
 import pkg/chronos
@@ -25,6 +22,7 @@ type
   MockErrorState = ref object of ErrorHandlingState
 
 method `$`*(state: MockState): string = "MockState"
+method `$`*(state: MockErrorState): string = "MockErrorState"
 
 method onCancelled*(state: MockState, request: StorageRequest): ?State =
   onCancelCalled = true
@@ -73,7 +71,7 @@ suite "Sales agent":
     onSlotFilledCalled = false
     agent = newSalesAgent(context,
                           request.id,
-                          slotIndex,
+                          some slotIndex,
                           some request)
     request.expiry = (getTime() + initDuration(hours=1)).toUnix.u256
 
@@ -83,7 +81,7 @@ suite "Sales agent":
   test "can retrieve request":
     agent = newSalesAgent(context,
                           request.id,
-                          slotIndex,
+                          some slotIndex,
                           none StorageRequest)
     market.requested = @[request]
     await agent.retrieveRequest()
@@ -156,3 +154,25 @@ suite "Sales agent":
   test "ErrorHandlingState.onError can be overridden at the state level":
     agent.start(MockErrorState.new())
     check eventually onErrorCalled
+
+  test "assigns random slot index for slot that is free":
+    let slotId0 = slotId(request.id, 0.u256)
+    market.slotState[slotId0] = SlotState.Filled
+    check isOk await agent.assignRandomSlotIndex(2)
+    check agent.data.slotIndex == some 1.u256
+
+  test "fails to assign random slot index when all slots filled":
+    let slotId0 = slotId(request.id, 0.u256)
+    let slotId1 = slotId(request.id, 1.u256)
+    market.slotState[slotId0] = SlotState.Filled
+    market.slotState[slotId1] = SlotState.Filled
+    let r = await agent.assignRandomSlotIndex(2)
+    check r.isErr
+    check r.error of AllSlotsFilledError
+    check agent.data.slotIndex == none UInt256
+
+  test "fails to assign random slot index when invalid number of slots provided":
+    let r = await agent.assignRandomSlotIndex(0)
+    check r.isErr
+    check r.error of ValueError
+    check agent.data.slotIndex == none UInt256