After 2 hours of reading this thread including my non-exsistant typing skills, a simple thought popped into my mind after I posted that should be rather easy to implement.
Just make it a requirement that anyone posting an item For Sale, MUST include real name, address and phone number.
I know this a really odd internet method, but I bet it will keep the scammers away and it will make it very easy for verifying to whom you are sending the $$$$.