When in the trading screen, if you try to use the scrollbar for your backpack, it does not work. You cannot scroll down past the current view. The most you can seem to get access to is 30 slots, even though you can have up to 46 slots of total inventory. Can anyone else with a kickstarter backpack try filling up their slots and see if they have the same thing?
Full Inventory (spread out, and notice arrow quantity.)
Trade menu (top of list)
Trade menu (bottom of list)