The new Auction House allows you to sell your gear for regular gold and real money. While we have seen microtransactions and/or downloadable content before in games, this is the first time players will be able to sell items for themselves for real money. Direct player to player trading. There are plenty of reasons why they wanted to implement something like this.
By giving a legit trading platform for real money they can control it and keep the amount of scams, hacks and other subvert activities to a minimum. Diablo 2 was riddled with bots and item sellers that scammed willing victims for their accounts. By giving these victims options for legit money trading they are hoping to reduce the amount of account hacks and other scams to a minimum. The question really becomes whether or not this RMAH can compete with the low prices that farming companies can offer.

Real Money Auction House (RMAH)
The RMAH uses real money as it’s currency, these vary from region to region and it’s not known which currencies will be included in regions. A player has a couple of free listings each week after that you need to pay a small listing fee to list new items. Blizzard stated that this was to prevent people from listing every item they can find. After the item has been sold there will be another fee to handle all the transaction costs. The money will be deposited in your battle.net wallet. They stated it will be possible to withdraw money from your wallet but this function will not be present at launch time for Diablo III.
Gold Auction House (GAH)
The gold auction house is the normal auction house, used in many games. You can buy and sell items for in-game gold. There are no listing fees or transactions fees associated with trading, player can trade as they see fit. Hardcore players will only be able to use this auction house, real money trading is not allowed for these characters.
What the effects on the in-game economy will be, is unknown as this is the first time something like this has been done. There are many ways real money can influence the economy that it’s hard to predict anything at this point.