I bought Diablo 3 about a year ago not because it's a good game, but more of an economic experiment. The real money auction house (RMAH) intrigued me as it was the only game at the time (and I think still is the only the game) that allows people to sell their loot to other people for real money without having to go through a third party like eBay.
I wanted to see if I could make an appreciable amount of money by playing the game whether through actual grinding of content or auction house flipping. To start things off I bought the game using money from selling a bunch of MTGO tickets. For those who don't play MTGO, tickets are sort of an unofficial in-game currency. You normally obtain them by buying them from the MTGO store but if you speculate on card values well, you can actually gain tickets which I did rather successfully (an aside, when I figured this out I financed an entire year's worth of drafts and online release tournaments with an initial investment of something like $20).