We had similar problems with a massive (50,000 + users) car forum where people were constantly getting scammed left right and centre. Big money sometimes on Turbos and expensive difficult to find parts.
We thought long and hard about it and ended up making the for sale section only able to post if you have signed up. Now i know you have to register to use the forum but VBulletin allows many tiers of registration. We allowed the for sale section open to view to non-registered users, (google bots etc trawl every part of the forum so this is a two-fold reason). However to sell through the forum you had to be signed up with by supplying valid details and paying a small contribution for the privilege.
Scamming dropped by 90% within a week - yes you can still get caught but limiting the site liability in that is half the battle. Its a difficult line to walk but i believe a one off small fee is definately the way to go
