EverQuest players would sometimes tire of the game, and decide to sell off their characters or virtual possessions at an on-line auction site such as eBay. When Castronova checked the auction sites, he saw that a Belt of the Great Turtle or a Robe of Primordial Waters might fetch forty dollars; powerful characters would go for several hundred or more. And sometimes people would sell off 500,000-fold bags of platinum pieces for as much as $1,000.So he did some math. Astonishingly, "It was the seventy-seventh richest country in the world. And it didn't even exist." So he wrote a paper about it.
You really ought to read the entire article, it's full of lots of other fun facts about the online worlds, including the dirt on the SimsOnline brothel chain (no joke), as well as some of the political and social problems inside the games as well as legal issues that may end up extending out of the games if one of the major online worlds were to be closed down because the game company goes out of business.