Would it be hard to make a Buyer/Seller Feedback percentage on your member card(it shows join date/post count so somewhere on that show the percentage). This is a really simple way to stop a lot of scamming because only like 1/5 of the members have made a feedback thread and anyone can just go right "legit guy" in there. Can slow the flow of pm's to mods complaining about being scammed allowing other places to be modded and the members have safety.
Also Agreement Threads are pretty useful as proof of a transaction happening. I'm pretty much 50-50 on this I feel that it's just added security, but not all that important since people can just forward the pm's. So if it isn't much trouble (idk if it cost more to open another subforum or whatever) that could be beneficial.