For a quick way to improve basic logistics of loot manipulation, expand GUI taller, even by 1 item's height. The more items we see at a time, the better. Then expand wider and add ability to categorize loot into 2 columns. BAM we just doubled the amount of items we can see in inventory. One column could be "stuff i need to always keep". If i set my "stuff i always need to keep" to include 1000 rounds of 12g shotgun shells, then i always retain 1000 shotgun shells even when i open a locker and say "put all" in there. It would only move excess shotgun shells to the locker, and not empty me out.
For improved trade though...yeah let's dig in!
A new GUI, that lists all open deals offered by players all over the map. A place where you go to accept a deal, and/or offer your own deal. Deals can be caps for goods, or goods for goods. You need to build your own caravan first, staffed by NPCs who run your goods to the other party. You find a deal you like, accept it, and away goes your private trade caravan. You get a prompt first: "Do you want to help guard your trade caravan?" Say yes, and you go along to make sure it reaches its destination, which is a random encounter on the map. There you meet the other player's trade caravan, also staffed by NPCs, and maybe guarded by that player. Your NPCs talk with each other and the goods are [silently] exchanged. Meanwhile, you players decide if you fight or make friends. Everyone knows in advance it can go either way, and if you don't guard your caravan, it can even get into trouble with normal random encounters.
If players stay friendly, then deal goes well - both caravans go back to map, now laden with goods from the other player, and return to base.
If players fight, somebody wins. They get to return their own caravan to base, still laden with their own goods, and both players get a dot on the map which marks where the losing player's caravan is. Both players have time to go back there and let's see who will get those goods! The launch of a trade caravan disallows bringing friends, but going back to the dot allows both players to bring whomever they can get to help.
The inter-player trade GUI could be a Pip-Boy upgrade, available by beating a tough quest to get a trade base. Base shows on map in random place - you don't get to site it, some of these deals across significant distances. Next, do some quests to rescue enough NPCs to staff your trade base, and finally by trading enough in towns that some NPC merchant somewhere says "hey you're pretty good at this" and BAM you can do deals now. When you make friends, click their name green in the trade GUI so you remember them. Click enemies red.