This got posted at work; it's a good entry level article to the secondary market of MMO games:
http://www.wired.com/news/games/0,2101,62929,00.html
What's interesting to me is how the argument of cheapening the experience for gamers that don't pay for game items is usually overlooked when discussing how it’s an economic system where players with more money than time can get ahead. The design/philosophical question of whether players subverting the system is good or bad is often assumed as good, and just dealt with. As developers and academics we somehow have this loftier goal of creating a balanced economic simulator, whereas for a lot of casual gamers it's just a game.
One thing not mentioned was EQ’s aversion to having virtual cash have a conversion rate to real cash, since that would create a financial liability where if a player lost their account or got hacked, they could request for the cash equivalent of their game assets.