Game Time Cards(GTCs) come from CCP. It is intended to basically be a way for people to play EVE by buying the cards in physical stores, as opposed to using a credit card to pay for the game. Players quickly realized that people would pay ISK for GTCs, so people with deep wallets started buying GTCs and selling them for ISK, which in practice was buying ISK with real money. CCP approves of it because they're making money off of the whole deal, and the person selling the ISK isn't getting any real money out of it.