Like many MMOs, EVE Online has a problem with players buying ISK from shady websites to short-cut the ISK-making process. On the other end of the spectrum, many players are great at making ISK but unable to afford the monthly subscription. To kill two birds with one stone, CCP created PLEX. Sixty-day game time codes purchased for cash can be converted into two 30-day Pilots License Extensions, which become items in the game. The PLEX can then be traded to other EVE players for ISK, creating a legitimate way to buy ISK without fear of being hit by the ban-hammer. Players with some ISK to spare can buy PLEX from the market and redeem them for game time, essentially playing for free. It's an innovative system, and until recently it was a completely safe market to deal in.