TalkBass Trade Gone Bad <---- straight up fraud and the member still participates in both the forum and classifieds. (EDIT: I see this user finally figured out they can change their name and did so to cover their tracks.)
Do you remember @iualum ? He was a fairly regular poster. I'm sure he still posts here but just with a new handle. He burnt somebody in the classifieds too. At least he didn't try to come back and act like nothing was wrong. Additionally, his case as not a high profile one like the one linked above.
Don't get me wrong, I like having some kind of feedback system I just wouldn't put much stock into it.
No system is perfect, but having a history on the other party at least helps point you in the right direction, in the majority of cases.
I only have one transaction in the classifieds, and the seller was good enough to trust me without any feedback. I proclaim to be a serious buyer, although sometimes not all transactions go through (due to circumstances like import issues etc.), but I completely understand when people don't want to perform the transaction based on lack of history. At least searching posts by the user gives you a better feel than nothing.