Nightmare !!
I had a simelar problem a few years back reguarding a set of expensive brakes for my M3, anyway the guy got my money and then decided to ignore my emails and phone calls......................anyway the only info i had on the guy was a first name and a mobile phone number ! so not much to go on for tracking him.
I ended up spending a full week on the internet trying to find out any information on the guy, and eventually i got a lead which i traced and it ended up being this guys dad !! (who has his own business), so from there i traced him to his current address, google mapped his house - very nice place with a CSL in the drive.
(I live 500 miles away from him in Scotland), anyway since he was rejecting all my calls i texted him during the day saying i have drove all the way down from Scotland, and was waiting outside his house (gave him his address which he never gave me ) and told him im waiting for another 2 hours outside his green front door to collect my money or brakes, and that id return in a week if i didnt manage to catch him in, to collect the money or items to the value of that from his place, anyway within 2 days the money was back in my account RESULT .................I never buy of people unless i can see posts where they have sold bits before and the buyer hasd been happy - Piston heads never again !!