I reckon a basic feedback sticky in the classified section could work quite well, nothing complicated, simply post up the user name and a brief description of the transaction and how it went i.e: “item as described, fast dispatch, fair price” or “Thieving scum bag, kill on sight!” that sort of thing…
the thread would be searchable simply by user name, no new or fancy changes to implement to the forum there and you could build up an idea of peoples trustworthyness from any posts relating to them, if there is no feedback against a username well you’re in no worse position than at present, but at least you know your taking a flier.
It would provide a bit of an incentive for people to do good deals and build up positive feedback (and keep the same username too), it also makes the Mods lives a touch easier as they only really have to watch 1 sticky thread for persistent Classifieds offenders rather than the sporadic Whinge threads that pop up all over the forum…
Obviously it wouldn’t be a definative solution, but surely a bit of an improvement over the current state of things, and very easy to implement…