I've sent all my payments to members of this forum through "Gift" or "Payment Owed", but then again I've only bought from reputable people.
Another big reason I think new members (100 posts or less) shouldn't be allowed to buy or sell in the marketplace. It would basically stop all scammers from signing up and ripping people off in there 10th post. Obviously buyers should use discretion when purchasing but that would help.
I don't know how hard it would be to set up for a forum, but there is a way to lock a member out of certain areas within certain parameters (100 posts or more)