It's an awesome idea - thought of it myself but soon realized I'm not a man enough to pull something like that off... You probably need to integrate some sort of escrow system to minimize scam - I suspect it's going to be a major problem considering your will be dealing with non reversible transactions.
Somewhere on
HashSwap.com there is a developer who has prototype of working crypto shopping cart (he uses LTC as default) and he was looking for some sort of partnership so you guys may want to hook up... Certainly this is not going to be an easy project - too many things can go wrong.
Good-luck!
Vadim