I completely feel for your situation. And I have 
a nice little story of my own that I will soon be 
sharing on this forum ^_~.

I haven't really had a problem with major dealers 
as much as say..... private sales. Only on a 
couple occasions have I wondered if the seller 
had fallen off the face of the planet. At the 
very least, a seller should provide some 
consistant communication! Email isn't hard, 
really quick, and you don't even have to go 
online to do it. (flash sessions anyone?) A 
simple "I recieved your payment, and I'm mailing 
the cel tommorow" works. I understand sometimes 
it is hard to keep track of tons of buyers, but 
when you decide to sell off a bunch of things, 
you have to accept that responsibility.

COMMUNICATION is probably key to online sales. 
IMHO

~Tara