How about some more stringent rules for the 'For Sale/Wanted' section?
The system they have here on this forum seems to work well.
http://www.avforums.com/forums/view.php?pg=trading_rules
Maybe implement some of the rules they have like this one:
5, PM AND EMAIL USAGE
PM and email may not be used at anytime in a live sale. All discussions including offers, acceptances and refusals should be shown clearly in the thread. Once a sale is complete you may use whatever form of external communication you like to finalise payment details etc. Your general geographical location and detailed specifications of equipment are not considered to be valid reasons for using off thread communications. It is the responsibility of both the vendor and the purchaser to ensure that all negotiations are conducted in the thread.