Why not have a "Disputes Section" below the For sale sections where disgruntled buyers (and sellers) can air their grievances about a transaction.

This can then be used for each party to communicate and will let others decide if they want to buy or sell to an individual.

The problem with these things is there are always two sides to every story, and you very rarely hear both.

Creating a thread in the "Disputes" Section should only be considered after all other forms of communication have failed.

Just an idea.