This is something I particularly enjoy about the Commodities Exchange in WildStar. A player can list a sell order of however many of an item they want (there's a little box where you type in the number), and it tells you the current price though you can change it if you want. A player can place a buy order that a potential seller can use to "sell now", if they want (can be used by buyers to try and get things a bit cheaper - seller gets guaranteed money right away). And if you're browsing the sell orders, you just specify out much of an item you want and at what price, and purchase. If there aren't enough of that item up at that price, the game will tell you, and you can adjust. There are some flaws in the CX user interface, but it works very well for all the items people like to sell in bulk but generally buy in individualized amounts.