You could have something like iTrader for feedback - which is similar to eBay's feedback system. Its used over on AVForums and works quite well. Each member has their own feedback rating in the classifieds section, which you are free to check etc.
However, its still down to your own judgement at the end of the day. You buy something through a forum and its at your own risk.
There are still a couple of things you could do to try and safeguard yourself from a member you do not recognise though. If its a recognised and respected forum member, who others have had successful dealings with, chances are you could probably trust that person and purchase. If its someone new etc you could:
1. If you are a 'regular' the seller should be ok with posting the item to you first, then you sending payment once you have the item.
OR...
2. You could always pay with normal PayPal and suffer the 3.4% extra yourself by adding it onto the total. That way the seller still gets the full amount, and you've basically paid a small amount extra to safeguard yourself. If the sale falls through, you can start a PayPal claim and retrieve your money.
Bank Transfers and PayPal Gift is like giving cash over the internet - dont do it unless its a well known member of the forum, or someone you actually can trust!
This system works on AVF because in the classified section all the deals have to be done in the threads until a price had been agreed, you can then finish the deal via PM. Then when leaving feedback you have to post the link to the thread in which the sale was completed.
So not sure this would work as well here as most deals seem to be done via PM.
