Having used a similar system in APB (and having ended up with a year of paid premium gametime well before the servers closed by using it) I honestly don't have any problems with the cash option on the auction house. That said I'm not much of a buyer but it would be nice to sell an item here or there and end up recouping the cost of buying the game. I'm not really sure this is something I necessarily want to see in other games but as other people have noted, people traded stuff all the time in the previous titles. This just makes it a bit more safe as well as EULA approved.
My only gripes with Diablo 3 are the lack of modding tools (meaning no cool total conversions or levels) or offline laptop play. Torchlight 2 will have those and it'll be a third of the cost, so it'll get my money first. I likely end up picking up Diablo 3 at some point, though.