I've used
www.accounts.com twice with no problems
They have live-chat help who will talk you through the process and status if you get "wtf where is my paypal money...i gave you my account info" paranoid like i was the first time i used them
Only use it if you're desperate though as they just buy low and turnaround and sell high, so you probably won't get much.
I got like $110 for a 70 shaman in blues for instance. And $22 for a lvl 40 scrub account i had laying around
Oh there is also
www.gamepal.com but they seem to give less