Right now, all of the shopping interfaces are set up so that every click on the up or down arrows immediately buys or sells the indicated number of items. This makes it very easy to over-click and wind up selling or buying too much, then you have to buy or sell in the opposite direction to fix the problem. Due to items having different buying and selling prices, this can eat into your profits.
What I'd like to propose is a shopping system that lets you preview your transactions before you finalize them. Rather than immediately buying or selling, your transactions are logged and added to a transaction window that lists all the purchases or sales you've requested. When you're done, you click "confirm transactions" on the window, and this finalizes all of your purchases or sales and adjusts your money and inventory appropriately.
If I'm not clear and you'd like a better explanation, I have (admittedly very rough) illustrations that I could share.
