[Swamper]

One 'quick' way to do this is implement your forums such that when a thread is 'closed' within the 'For Sale/Wanted' forum you can make it a rule that whoever bought it the item can post on their experience...

1) a "For Sale" thread:

You want to see how the feedback on Mr. Jones - you click a button that does a mysql query that looks in that forum for all threads Mr. Jones has started (selling stuff), that are closed, and extracts the message written by the last poster in the thread (the buyer)

2) a "Wanted" thread:

Same as above but the person that started the thread is also the one making the last post regarding the purchase - in their last post they can put the Username of the person on the forums they bought the item from, and in their message list any comments.... etc.

This way you would prevent others (who weren't directly involved in the exchange) from just leaving random comments on individuals in that forum.