Ron, have you timed yourself while trying to buy/sell the amount you want.
I don't have to time it to know that any reasonable amount (2000 or less) takes far less than the simple act of moving my right hand off the mouse and finding the number pad. Especially since I'd first have to click on the special field, then type the numbers, then click on the buy / sell button.
Yes the current system is streamlined in the sense that it is simple to use, but I don't think it is quicker to use than other options
If you actually read (and applied) what has already been mentioned, you'd know that you can shift-click, so for any reasonable amount its quicker just to hit the button. I mean, come on, the shift click is all of a pinky's move away!
The
only reason for what you're suggesting to be superior is if you're micro-ing in smaller-than-100 units for some reason, which I don't think you'd have time to do very much, if at all, or if you need to do massive amounts of buying/selling.
And if you have the resources to pull that off, the games already won.