I don't think we need a feedback system per se, but we need a sticky at the top of the classifieds section with offenders. Inclusion to the list of offenders should not be taken lightly, but there should be some sort of published & acceptable norms that must be violated before flagging. It should be a moderator decision ultimately, but with feedback from members.
There should always be the option of being removed from the list, pending moderator decision and whether or not the person can justify the original offense, due to emergency or something like that.
Likeise, a list of successful transactions could be made. For instance, a member emailed me the other day w/o PMing, and since my email address is public on APUG, I was at first concerned. He's not a member I see post often (Jim McDonough), but a few google searches confirmed that he was probably a reliable member. Point being, this classifieds section is really rife with opportunities for theft, and we're only fortunate that most people here are trustworthy and honest.