I have an idea. How about allowing each user to post their own items for sale.
That sounds good, however I am designing this mainly for admins of sites who sell products, and are looking for a easy way to do it dynamically.
But your idea is very good, and it would certainly add a lot to this. What I think I will do is make it so you can toggle the abilty for users to post products from the admin control panel.
Also, that might come in handy if you had a forum based on a game like Diablo 2, users could easily post items they are trying to trade/sell, without having to sort thru a messy trading forum.