I've got a few ideas for features on the auction house.
The first is that I'd like to be able to see the vendor value of items when deciding whether to buy or sell. Right now it seems like only way to see vendor value when looking at items to buy is to click through to the encyclopedia entry for the item. It's even tougher when selling.
My second idea is that I think it would be cool if you could partially buy out an auction. For example, if somebody is selling a stack of 1000 of I'd like to be able to choose to purchase just 10 of the item.
My final idea is that I'd like to be able to purchase multiple auctions of the same thing without having to click though several different screens.