If you KNOW you can sell the item you would not need to do that, or could even craft the item on the spot to fulfill the order. (nice rpg elements there) (maybe be able to put a lock for one hour on a crafted item you wish to make by depositing 10% of the worth of the contract as bond, but ensuring you are the one that can fullfill it before you craft it, so you wont get stuck with an item that is useless for you just because someone else crafted it a few seconds earlier )
I think this Ah change would result in lower average prices (since if a person knows he is going to make a profit he will often sell below the max of what he would like to get).
the classic auction house would then become the place where people try to get high prices for rare items in high demand that have no high offers at that time, hoping for an impulse buyer (or someone who does not want the money tied up unless he can be sure to buy something)
Obviously there should be a bottom price for listing items you want to buy, similar to the standard price the AH thinks you should sell something for (whats it about 1.5 times the vendor price? I forgot) to prevent people from clogging up the lists with trying to buy things below vendor price.
Also a nice option would be to be able to list only stacks of a specific amount or above. This would help to reduce the clog someone would have to go through if they want to sell a large amount of mats. This option would also be a boon on the buying part of the AH, allowing you to skip the 5 pages of single pieces of leather or cloth.