I would definately track down his phone number (or his mom's number) and talk to him that way.

I had an issue with a seller at UpstateGames.com just over a year ago (absolutely piss-poor communication from him). After a few weeks of zero contact from him and no response to my emails (it was a VERY expensive item) I did some checking around and figured out his full name, home phone number, fax number and home address. I used his full name in every one of those methods of trying to communicate with him, until he finally got back to me (seemed pretty freaked out). Harsh, but it worked.