It would be nice if there was something similar to the bazaar item search, but for orders, where it would give you the option automatically enter previously entered quantities. This would be a huge help in scenarios where you're buying large amounts of items while actively trying to undercut other people undercutting you. And in general, it would be nice to not have to interact with sign-based input.
Additionally, there could be a button for buy orders that would enter the maximum amount you can afford, and for sell orders a button to sell everything but 1 item (so you can quickly refresh the orders).