Cmdrslack (who's a gamer and a lawyer) says there's three ways Blizzard could be handling the case. First, they could be filing under CAN-SPAM law, claiming that even though the in-game mail is never actually leaving Blizzard's servers, it's still illegal spam email (first of all because it doesn't identify itself as advertisement). The second possibility is an much older tort called "trespass to chattels," which means that Blizzard could be saying the gold seller is unduly using their servers, bandwidth, and game properties to advertise their own business. That, says cmdrslack, seems most likely, because there's precedent for it, and Blizzard can easily prove that the spammer has been working on their servers for a while.
Finally, Blizzard could also simply say the spammer is violating the EULA, which they definitely are. More likely, as cmdrslack says, they're using a mix of all three cases to show the spammer is wrongly interfering with their business. (Strangely enough, says Tobold, the one thing Blizzard isn't suing the gold sellers for... is gold selling.) But cmdrslack closes with the same question I will: Seeing as the spammer is based in China, and Blizzard is an American company filing in US Federal Court, just how are they going to enforce the ruling when they win?