If you mean the business of selling time codes and recharge cards for in-game currency (e.g. time card wow 60 days can be bought for 15 000 gold on the server, azuregos) he usually works as follows:
The crooks (hackers) are stealing/chardet/yet somehow illegally get time codes/cards. Then they exchange these cards in-game currency. Then in-game currency they exchange for rubles. In this way they launder stolen money. That is why blizzard is not highly recommends buying in-game currency for real money or to purchase scratch cards for in-game currency, because it turns out that you're aiding the crooks.
Of course there are more options for buying gold and its subsequent resale more expensive, but generally speaking it is not very profitable, well, just struggling.
If you are interested in the business of selling time cards for real money it works just buy from a vendor (such as blizzard) on the time card and sell for more than bought, the usual scheme for any trade.