I have a relatively simple suggestion to make trading much more simple:
trading window "enter quantity" sub-window.
This new small window appears each time we try to trade (or move some object we have to some other window) when amount of objects of this type exceeds 5. This window should have:
on the left: icon and name of the object we're transfering
on the right: quantity that we want to transfer
in the middle, between left and right (or below): slider, by pulling slider to each side we change amount (minimum is 1, maximum is whatever we have)
lower left: cancel button
lower right: OK button

Example: I have 196 Blue Shards. If I want to trade, sell to vendor or offer on auction house 150 of my blue shards, I have to click on the icon 150 times in a row (!!!). My suggestion would decrease amount of clicks to 2 - one click on the slider or on the area where number is (so I write the number myself, same as with energy and crowns in trade window), another click for OK.
If we have up to 5 objects of same type (5 ghost bells), it would save time to click up to 5 times in a row.

Window size should be big enough to make it useful, small enough so it doesn't cover part of inventory so I don't click any button by accident. Clicking outside (on reflex) should do nothing, the rest of the window should be grayed out (same as with trade requests and invites) so player could see what window has priority.

Once in a while every player makes a trade with large number of objects traded. This will certainly make player's life easier.

PS: Thanks for reading suggestions forums and implementing some of them :)
PPS: If you liked this suggestion, send mail with 5 CE to knight Round-Shinigami