The obvious problem is how to deal with scammers.  IE those who sell an item but do not actually deliver it.

This can be remedied in various ways, E-bay uses a rating system, which is a good idea.  The problem is that with the complete anonymity of bitcoins scammer sellers would simply make a new account.  Making accounts in some way not anonymous, and in need of verification, could, perhaps, be a solution to this problem.