This would speed up trading with retainers a lot. I like items to stack on retainers. Currently I have to search for the item, then select retainer, open trade and trade that item s/he already had to make it stack.
If the trading screen items had icon or border showing if the other inventory had that same item, I could trade all the items that we both have on one go. I would only have to go through the retainers once to 'sync' all the items.
I would think that this is easy enough to implement and it would speed up trading with retainers tremendously.

Please make this happen!