Blizzard announced on their forums that they were sueing the gold-selling company Peons4hire in a federal lawsuit, but failed to say on what grounds. They only mentioned they asked them to cease and desist in-game spamming. Intelligent commentary from my favorite lawyer.
The one thing Blizzard *isn't* sueing the gold sellers for is for gold selling. Because you can't go to court on that issue without tackling the difficult question of whether something like virtual property even exists. And if the courts would come to the conclusion that it does, based on the existing trade in it, that would open a vast can of worms which could kill the MMORPG industry. So you can't sue the gold sellers for selling gold, and have to get them for spamming. Reminds me of Al Capone, who ended up in jail for tax evasion, not the numerous crimes he committed.