Inventory control is included in the powerstore.
The way the powerstore does inventory is this:
1) You enter the inventory in stock when you insert or update the product. It stores the data/time when you updated in a separate field.
2) In the normal checkout process your orders and details are inserted into the database.
3) In the catalog we use a complex SQL statement to automatically subtract the number of products sold since the date when it was last updated by comparing the products update date and the order date and subtracting as many as appear in the order details.
As you can see inventory control in this case doesn't even really involve the cart. It just involves writing a complex sql statement and taking advantage of a stock field stored in the products admin back end.