In Eve, the in-game currency can not be traded for real world money (well, without risking being banned from the game by using some shady sites). It is possible to go the other direction, though. You can purchase a "PLEX" for real money, which is an in-game item that you can redeem for a month of game time (a "Pilot License Extension"), and that item can be sold on the market in-game for in-game money. The amount of in-game money you get depends on market conditions in the game though. Some hard core players make enough in-game money that they support their subscription by purchasing these PLEX, essentially paying someone in-game money in exchange for that person paying their subscription fee. There are the regular supply and demand (and potential market manipulation) factors that come into determining just how much PLEX sell for on the market, though.
CCP decided to create that sort of exchange to undercut 3rd party "gold farming" type operations, and by having the in-game money come from other players in a market transaction they avoid the in-game inflation that would occur if they just gave you in-game money for cash, created out of thin air.