CCP (the makers of EVE) are really tough against in game currency sellers, however they also allow people to pay for their subscriptions with in game currency. This is done using game time cards, and the providers for the time cards for the people who are buying with in game currency? Other players.
Basically you buy a time card with real world money from a supplier, then post a listing on EVE forums (or you can advertise in game, or anywhere else I guess), with what you are expecting to get in in game currency (the average seems to change with the economy in EVE, current average is 600 to 650M ISK an amount which is close to getting you a smaller capital ship minus fittings), someone will usually respond soon, you then HAVE to use CCPs official escrow system (to protect both parties, they monitor large cash transactions where no goods or services have been traded), where by you enter the time cards code the buyer and price, the buyer then has to accept it and if they do you get the in game currency from their account and their account gets the time allocated to their account.
A rather round about way of doing it, but the price of the time cards in game currency is pretty much decided by the in game economy and what people think is a fair price. There doesn't seem to be much moaning from players who don't do it, plus it has the advantage that if you can generate enough in game currency you can essentially play with no subscription fee. Of course to do that (at the moment you need 600-650M ISK every 60 days) you would have to spend a huge amount of time playing EVE (from my experience, I stopped playing a while ago, but have friends who still play and am informed the new wormhole exploration is very profitable).