I think you guys are confusing two different systems.
1. Post Reputation system - Where you can give someone positive or negative reputation points based on a post they made. This is simply an option you turn on in vB and has been tried on the forum before. This won't really do anything for stopping scammers.
2. Transaction Feedback system (like iTrader) - This allows you to leave feedback for specific sales or purchases with a seller/buyer. This is an add-on you can install in vB. From past experience, it can be helpful as long as everyone uses it.