I've been using http://www.offgamers.com for a long time and never had any problems. I even have a discount from buying so many accounts.

Normal WoW Auth (EU): 9.02 euro
TBC: 9.49 euro
Battle Chest: 18.04 euro

It's cheap and the delivery really doesn't take long, about 5 minutes to get 5. At least , since I'm a returning customer they don't need to verify me.