Hardly anyone buys anything unless they think they are getting a bargain. When selling this means undercutting the lowest buyout number in the AH. But how low do you go? Why am I forced to spend time constantly lowering my item buyout until it sells? The other way to sell is to set a low start bid to try to attract attention, but the danger is that your item (unless it is godly) gets lost in the forest and someone snipes it at below fair market value. When buying items, why pay more when you can snipe at the last second, or play the refresh warrior looking for 1day12hour items that are under priced?
So how do you more conveniently convey the fair market value of items? Simple. Add the ability to search "recent items that have sold". And so as to not overburden the fragile AH, you don't have to keep records of ALL items that have been sold, just RECENT ones, maybe 2-4 weeks worth of data. Yeah this would end a lot of the "Great" deals people get off of uninformed sellers, but this would also allow people to spend more time playing the game instead of the AH.