Similar to F2P games it will have an cash shop to buy and sell items for real money. You can also cash out your earnings from the auction house straight into your bank account. It may even have free-to-play option too like WoW does now for its marketing strategy but no word yet.

For more screenshots and official quotes coming from Blizzard go here:
Diablo III to Incorporate a Cash Shop

Notice the items on sale are in $ dollars.