diff --git a/design/sales.md b/design/sales.md
new file mode 100644
index 0000000..c109898
--- /dev/null
+++ b/design/sales.md
@@ -0,0 +1,47 @@
+Sales module
+============
+
+The sales module is responsible for selling a node's available storage in the
+[marketplace](./marketplace.md). In order to do so it needs to know how much
+storage is available. It also needs to be able to reserve parts of the storage,
+to make sure that it is not used for other purposes while a contract is running
+or being negotiated.
+
+    ---------------------------------------------------
+    |                                                 |
+    |    Sales                                        |
+    |                                                 |
+    |    ^   |                                        |
+    |    |   |    updates    ------------------       |
+    |    |   --------------> |                |       |
+    |    |                   |  Reservations  |       |
+    |    ------------------- |                |       |
+    |          queries       ------------------       |
+    |                           ^         ^           |
+    ----------------------------|---------|-----------
+                                |         |
+                  availability  |         | state
+                                v         v
+                   ----------------    -----------------
+                   |     Repo     |    |   Datastore   |
+                   ----------------    -----------------
+
+The Sales module keeps track of storage reservations in its internal
+Reservations module. It keeps a record of which pieces of reserved storage
+belong to which storage contract. It queries and updates the amount of available
+storage in the Repo. It uses a Datastore to persist its own state.
+
+The Repo exposes the following functions that allow the Reservations module to
+query and update the amount of available storage:
+
+    Repository API:
+      function available(): amount
+      function reserve(amount)
+      function release(amount)
+
+The Datastore is a generic key-value store that is used to persist the state of
+the Reservations module, so that it survives node restarts.
+
+    Datastore API:
+      function put(key, value)
+      function get(key): value