I was thinking : maybe instead adding trade slots which won't reduce much drag and dropping (touch screen annoyance ) but reduce amount of trades here is an alternative :
First off we need a highlight option (ctrl+RC or icon on mobile activating highlight mode) then we can select multiple items let's say 40 stacks of trophies.
Now have those highlighted we right click someone and choose "transfer"
Another person gets pop up :
"*Player name* would like to transfer X (40 in this case)
item(s) to you "
*Accept/cancel*
Upon accepting boom ! All stacks in your possession at once.
This would be perfect alternative for "alt feeders" while regular trades still will be used when we expect things in exchange.