From Blizzard's New MMO Wiki
Real Money Auction House
Diablo II taught Blizzard that people found a way to not only trade useful item drops to help fellow players, but to also make a bit of money on the side. In more recent times, this process has become known as Gold Farming, and rather than ignore, or fight the trend, Blizzard have embraced it for Diablo III, offering up active auction houses for items, split into currency regions where they, of course, take a slice of the pie, but you can freely sell items for real-world money to then pocket or use against your Battle.net account for purchases or usage with other Blizzard products and services.
So the basics are, it’s integrated into the game client which means you can trade completely within the game itself. You’ll be able to sell items, gold and components with auto-bidding or via instant buy-out (so if you’re familiar with the WoW auction house you’ll have a sense of how this works). Auto-bidding is a new addition though, that will allow you to set a certain price for specific items you’re chasing, which means you won’t always need to be online to participate in auctions.
Searching for items is a simplified and easy affair thanks to a lot of smart search functionality. Given the game works specifically off of a random item generator (so you never know what you’re going to get), you’ll be able to search for affixes and items integral to your chosen skills, class or load-outs for maximum customisation, and to really give the trading system robust purpose.
The game is going to require you to be always online for full access, but this will allow your auction house movements to be immediately linked across your entire account, and there’s an in-game “Stash” box where all your items bought or found will go, which you can then uitilise across any characters you have with your Battle.net account. It’s all instantaneous. Moreover, all transactions within the auction house are part of a secured item transfer system, meaning you no longer need to arrange to go into town and meet your buyer and drop his gear in front of him in a hope he picks it up before someone else does. All trades will also be conducted with anonymity (so you’re invisible during trades, essentially) to maintain the secure aspect of what Blizzard are attempting to do here.
All that said though, you don’t have to use the real money auction house, and it was revealed the team were also offering up a separate gold auction house, where you’ll use the game’s own currency to buy items. And for the Hardcore mode players out there, you won’t be able to use any items from the auction house in that mode.
Auction houses will be split into regional currency, though it is possible to participate in other regions, you’ll just cop the appropriate fees applied to conversion rates and the like.
As for how the money end of things works, there’ll be two different fees - a listing fee, and then a sale fee. Blizzard’s Rob Pardo mentioned in a interview for AusGamers.com that they weren’t ready to go into specifics just yet about what sort of dollar value you’d be looking at for both fees, but assured us they’re not looking to pinch your pennies, and that it would be a nominal figure for both and fixed, flat-rates, to boot. He did also mention though, that they were toying with the idea of allowing for a certain number of free listings per week as both an incentive to participate, and general loyalty reward.
Speaking of cash, cashing out of your earnings from the auction house can be done in two ways. One is to simply roll your earnings into your Battle.net account balance, which will obviously net you the highest return, because the second option, which is to physically cash out, will likely set you back extra fees as Pardo revealed they’d be going with an unannounced third-party payment provider.