Updated fature

How about adding on a button in the inventory/stash that has an "empty all" feature which places all items in stash into the inventory or from the inventory into the trade screen. That would help out when trading gems and runes that are traded in mass quantities at times.|||Nice idea. Perhaps you should post this over on the Suggestions Forum.
How about this:
Instead of calling the button "empty all", call it "Transfer". Then have a combo-key such as [Ctrl], [Alt] or some other key that's not already in use, where you can hold that key and click various items. Doing this would highlight just those items you combo-click. Then when you click "Transfer" it would transfer just those items.
The problem with an "empty all" button, at least for me, is that I already have a lot of items in both my Private Stash and my Inventory. In one case, to "empty all" would try putting everything in my PS into my Inv. Because it would not fit, a lot of it would end up dumped on the ground, forcing me to pick it up, which I couldn't do because my Inv would be filled with what could fit. In the other case, my Charms, Tombs and Cube would transfer to my PS, which I don't want, or it would be dumped on the ground, as in the previous case.|||I like the idea of a new ability to mass move things from stash/inventory/trade instead off one at a time. It can be so time consuming.
For me, the move all thing would never work though, I make use of much of my empty spaces!