This article is just too odd... It discusses the legalities and taxes associated with MMO's and barter of good for real world cash or items. While it mentions WoW, the main concern is Second Life which actively promotes the secondary market. Still a VERY interesting read!

Virtual worlds, including massive multi-player on-line role-playing games (game worlds), such as City of Heroes, Everquest, and World of Warcraft, have become popular sources of entertainment. Game worlds provide scripted contexts for events such as quests. Other virtual worlds, such as Second Life, are unstructured virtual environments that lack specific goals but allow participants to socialize and engage virtually in such activities as shopping or attending a concert. Many of these worlds have become commodified, with millions of dollars of real-world trade in virtual items taking place every year. Most game worlds prohibit these real market transactions, but some worlds actually encourage it. Second Life, for example, grants participants intellectual property rights in their creations.

You can check it all out here: 'Stranger than Fiction': Taxing Virtual Worlds.