They should just do what WoW does and make it so when you buy a certain quantity of a stackable item, it automatically selects the lowest priced items on the market, gives you a price for those items, then lets you decide whether you want to buy them. For example, let's say you want to buy 50 iron ore. You select iron ore, type 50, and the game will automatically select the 50 cheapest pieces of iron ore available. Say there are 20 pieces for 2 gold each and 90 pieces for 3 gold each. It would select the 20 2-gold pieces and 30 of the 3-gold pieces, and give a total price of (20*2)+(30*3), or 130 gold.