There is a market for leveling people's characters. It's illegal based on the rules Blizzard, the game manufacturer puts out there, but it still happens. Basically someone levels the character to max level, then sells the account to someone, and starts over.
It is a relatively older game, but it's so popular it's still the most profitable video game out there I think (Call of Duty franchise is also up there). Because it is a MMO and the content is always updated, people continue to play it.
PowderHound and TreeNinja