I have wondered about that myself. Most games that allow you to buy things have a buy all/sell all option. if "ALL" means all of that commodity you have in your hold to sell or all the hold space you have available to buy, it would be great.
It only needs a small button at each side of the selector. You can click the + or - to do one at a time or hit the button beyond those symbols to buy or sell all. If you don't have enough credits to fill your hold, it buys as many as you can afford.