How's about a "looking for item X" posting?
You'd list an item that you want at a given price and then put down the gold into the AH - set a timer for anywhere from a day to a month - people browse the listings and if they like your offer then they place the item into the slot, hit SELL, confirm they really want to sell it to you and you're both happy.
You could also put up a quantity that you're looking for at a certain price, say, buying stacks of ruin cloth for 3g each and you fund the AH with 30g so that people can sell you up to 10 stacks of ruin cloth. A fraction option could even be added, although buyer would have to control that to keep some idiot from selling you all 10 stacks as singles (unless you were dumb enough to allow "quantities as low as ").
The looking to buy feature could be vague or specific - for example, chanters could set up a fund with 100g and buy all level 50-60 items for 5g each, or you could request items by the first or second half of their names, thus making for it easier to get "of the bandit" gear or to collect a specific set of gear.
This would help get more items to people who needed them as selling would no longer be as much of a chore since people would be able to dump stuff on the "looking to buy" crowd rather then merchants, it'd give folks trying to work certain skills a chance to get mats cheaper (I can hope I sell this at 7g a stack in two days or just dump it on Bob for 5g a stack now).