This has been discussed regularly on here, there are legal issues as well as who would run the feedback system and intervene in disputes.
You might have noticed that more people are posting details of successful transactions, and thanking the other party for the item, so an informal system is already in place.
I find the pub test useful.
If I were sat in my local pub, and a guy I did not know tried to do the same deal, would I take it?
If the answer is no, then walk away.
Considering the number of sales on here, fraud is a tiny percentage of total sales. The car park at a UKAHFT shoot looks like a car boot on occasion!
If you are unsure about a persons rep, then maybe PM or email a moderator.
Richard