...why didn't the seller give you the username and password for the account originally created with that CD key? Nevermind that the EULA specifically states that you aren't allowed to transfer accounts. Yes, I realise that there's a double standard there saying that the media can be sold, but the account can't be transferred. That's where the real ambiguity in the EULA is. It should be that both can be transferred, or neither. (Personally, I'm for neither, since level 60 noobs are no fun).

In any case, if you never got the username and password because the seller are still playing the game on that account then you have no leg to stand on, and you were pretty well ripped off. Blizzard has repeatedly stated they aren't going to sell CD keys alone. If they just forgot to give the username and password to you and they aren't playing anymore, you should be able to get the information from them and just use the existing account. This is a violation of the EULA, though. See previous comment about EULA ambiguity.