I like the idea and have thought about it many times myself, but I think it's hard to police. If person A complains about person B, but person B says it's not true and wants the comments removed, what should be done?! Maybe it isn't true and the comment should be removed .... but then again maybe it is true and should be left to warn the community? Who makes these decisions?
So maybe an alternative is allowing reviews, but only positive comments - then any negative comment is removed, true or not. Who's to say a swindler doesn't get friends to make false positive comments? Or popularity makes some sellers appear more legit than others?
So maybe an alternative is that we just say "go get'm!" and leave all moderation out of it (aside from language, etc), so buyers can decide for themselves if they want to risk doing business or not? I suspect people would still complain to moderators about it and if we just let things go, I wouldn't be surprised if at some point the law gets involved when one person sues another for slander and involves the forum.

Overall I like the idea and in an ideal world where everyone is honest it'd be great. However, in the real world it just strikes me as hard to implement.