What blizzard does not have a patent on is the use of a virtual currency. If all Blizzard can prove a gold selling site is doing is agreeing to exchange virtual currency for real currency, this is not violating Blizzard's IP.
The company I work for is a good example. We develop asphalt technology and license customers to use our IP to manufacture seal coat. These customers can make product using the recipe we sell them, and profit as much as they want to, by making the product and selling it to end users for a higher than cost amount, or by winning a bid on a contract to apply the product. The licensees are free to sell the product to whoever, for whatever,even though we own the IP. The only thing licensees cannot do unless they wish to break the law, is take our product formulation, and market it as their own. Anything less than that is not a violation of our IP.