Diablo 3 is partially a cloud based game. All the content is not installed to your computer, some is only on Blizzard's servers in order to prevent cheating/hacks which would directly be about items. Blizzard are doing everything in their power to prevent such a thing from happening since they are relying on the Real Money Auction House's success in order to make absurd profit.
That could be destroyed if the items/economy was tampered with like it was in Diablo 2. The entire game is basically built around the RMAH, even at the cost of the quality of the game. It's for this reason there are no trade channels, they don't want people to be able to trade directly and avoid their Auction House fee which (if it's the same as the in-game Gold AH fee) is a whopping 15%. That's robbery, I don't use Ebay but I can't imagine them charging 15% on every transaction.
I wouldn't be surprised if they actually make more profit from this system than monthly fees like in WoW. Either way I can't imagine it lasting long if they don't revamp the entire game, the flaws and poor programming in the game are endless. There are more sacrifices of quality in order to be built around the RMAH such as the obvious lack of offline play, etc.
People are even speculating that the Auction House does not show an ID or name of any sort in order to make it easier for Blizzard to just conjure up super rare items and sell them there. I would be surprised if they didn't do this.