Better yet, how EVE does it. You list 99 pieces for, say 10 gil each, which is currently the lowest on the market. Someone comes in and wants to buy 20. 20 of your 99 are sold to them at 10 gil each, with the remaining 79 still remaining up for sale. Items posted for sale at the same price (ex. someone else comes in and posts some for the same 10 gil each) are served in FIFO order, while requests to buy items are filled lowest-to-highest. If someone tries to buy more than is available at a given price point, it buys the remainder at the next price point up. For example, if you have your 79 for sale at 10g each still, and Person B else comes in and posts 5 at 10g, and then Person C posts 5 at 9g each, and then Person D posts 99 at 11g each. Person E comes in and tells the MB they want to buy 99 of them. The first 5 come from the lowest order, Person C's at 9g each, 45g total. The next 79 come from your order, since it was the first posted at 10g, and the next 5 come from Person B's, also at 10g, 840g total. The remaining 10 come from Person D's order at 11g each (110g total), for a total cost of 995g, and the new lowest order is the remaining 89 of the original 99 posted for sale by Person D.
Basically, full commodity market functionality. Doesn't make sense for things like mounts, minions, housing items, gear, etc, but for anything realistically intended to be generated and purchased in bulk (essentially anything in the consumables, crafting materials, dyes, materia, etc), it makes both purchasing and selling them much easier. It also removes the need to structure one's sell order quantities around likely buy order sizes, and if the MB interface auto-fills the current lowest price for that item, largely negates the "oh, hey, the current lowest sell order is for 3999, I'll post mine for 3000 so it'll sell faster!" silliness that can cut the price of a good by 80% in less than an hour, and then see it bounce right back up half a day later. Reducing market volatility in commodities is, imo, a very useful side-effect.