It is definitely a quality of life issue, and a poorly communicated one at that. My suggestion would be to just us a single menu for selling that includes your camp inventory and uses the highest CHA team member as the merchant (doesn't matter if they are in the party or not). I'm not sure how the camp storage works with multi-player, but I am assuming that each player has their own. If not, there would need to be a tweak to that system when there are multiple players selling goods.
