For me, I use PlayerAuctions.com. It is a very safe site to use, you pay the money to the site, then you receive the login information + secret answer etc, you make sure everything is in order, changed passwords, and change email address. After it is completely yours. THEN the player receives the money for the account.
I have managed to get cata accounts from as cheap as 50 dollars. A cata account with an 85 might be a bit more. I think the cheapest there is 85 dollars at the moment. Which is still really cheap considering the cost of all the expansions is like 80?