not only items, but in-game currency.
This site alone offers fifteen massively multiplayer online game currencies to be purchased.
So do the math:
15 unique games X approximately 15 - 30 different servers per/game. That's how many unique worlds they're offering to sell currency to.
It's a HUGE market.
And IGE isn't the only "company" that does this. There's definite competition in each game.
These company employees that rake in all the gold/platinum to sell online are generally referred as "farmers".
They'll sit in an area and repeatedly kill the same creatures if they know that they'll get nice items/currency to sell.
It's a definite pain in the ass to players. And it's something that Game publishers and mangers are constantly at war with.