I wouldn't worry just yet. Situations like this are usually the result of poor / mis- communication. If I were you, I'd:
The seller's probably just busy, and may be logged on permanently - so no telling if the user's actually online (unless, of course, he's posting). At any rate, best course of action, IMO:
[1] phone call (you do have the seller's full and complete contact information, right?)
[2] email
[3] public request for seller to contact you (no accusations, no airing of grievances, just a request for contact)
[4] let us GMs know who it is and we'll have a chat with the seller
Seem reasonable?
Regards,
Brian