The sci-fi game EVE Online is set apart from many other massively multiplayer online games by its economy. While there are currently problems with the game's market due to a serious exploit, the economy isn't getting reset anytime soon. The currency of EVE's virtual economy is the Inter Stellar Kredit (ISK). As in the real world, money can be power in EVE's 'New Eden' galaxy. Most online games built around a virtual economy will ultimately need to deal with the real money trading (RMT) issue -- either make it a bannable offense or support and regulate it in some way. CCP Games figured if RMT is happening whether they want it to or not, they may as well have some way to regulate it.
EVE Online has a few legitimate ways to bring real world money into the game. Game Time Cards (EVE Time Codes) can be converted to ISK without risk of getting your teeth banhammered in by CCP Games. They offer a secure way to sell game time to players where neither party can be scammed, and more recently a way to turn game time into an item (PLEX) that can be sold on the open market in-game. When a player buys that item, he or she extends their account subscription. The upside for some players is that they can actually pay their subscription fees in the ISK they've earned in-game, for others it enables them to buy a faction battleship or a number of other virtual items.
But... some people just go too far with all this, and buy ISK in huge amounts. Even worse is doing so outside of the EULA-safe methods. This was the case with an EVE player in Denmark, who paid 100,000 Danish Krones (the rough equivalent of 13,500 Euros or USD 19,000) for currency from a shady ISK seller outside of the secure system, and was ultimately left with empty pockets, even after the case ended up in court.