atm when the caravans come, you have to select each time again the stuff you want in your depod, and when trading you have to AGAIN select what you just selected 10 seconds ago
i suggest a new form of designation property (designation property like dump, forbid etc)
namely "designated for trade" with t/T shortcut, or some other name that works
any "designated for trade" item will be handled like dump, but dwarves will try to store it in stockpiles instead and not touch it further
once the caravan shows up any "designated for trade" flagged item will be hauled to the trade depot automatically, the old system can be used for making adjustments to the desired goods in the depod (flag new items as "designated for trade" or flag some items as one does now for only this trade in particular or de-flag them if desired)
also i suggest that depending on the traders level more or less items can be traded at one time lets say for example:
no skill
OR or NEITHER
> 10 dwarfbuck in total
>1 item in total
Dabbling
OR or NEITHER
> 15 dwarfbuck in total
>2 item in total
etc... other values could be used but id keep it quite low at first
for it to be able to be traded, if both conditions of your skill apply you need more skill, and the trade cannot be completed
this for the simple reason that trading is otherwise just "select all of worth" on sell list, "select all of interest" on buy list and trade,
now with this the trade is a minigame and the best deals within the constrains of the traders skills should be made, combining items in a trade, purchasing those 2 mediocre value items you cant use for 20 crafts each just so you can trade it back to the trader again for that anvil of 100$ together as to be able to sell and buy as much items as possible in one go so that you can get that anvil even with novice trader skill, but you will have to put some work into it
the higher your traders skill goes up, the less work you need to do for him and the easier you have it on just buying large lots of items
additionally an extra "additional options" in stockpile namely "allow designated for trade" that is automatically engaged like the other additional option fields