The 1st thing I'd want to make it work correctly is a "check routine call" for current inventory number to make sure item(s) in-stock. That way you're not trying to sell something you don't have (policy problem).
Sounds like a site that's keeping track of the current inventory on an item is part of the solution. Who do we know that's doing that service on things we sell? Hint: Not eBay.

Danno