I just traded an arsenal to a new player in co-op and here's how...
I have all of the weapons open. Only the host of the game can retain weapons. Join a co-op game.
1. Go to any outpost and select the loadout you wish to gift.
2. Kill the Golden path members hanging out there.
3. Select one of the weapons on the ground to drop one in your inventory.
4. Have your co-op partner pick it up.
5. It is now unlocked in their trading post.
This does not work for preorder weapons. It does work for signature weapons.
You must pick up a sidearm to drop a sidearm. Have your co-op partner equip one to a slot other than a sidearm and then pick up a weapon to drop the sidearm from that slot. There is now a sidearm on the ground to replace the one in your sidearm slot that you wish to drop.
If you start a new game and someone gifts you all of the guns you will have early access to all of them. Now go and devestate Pagan's forces in style.
I already finished the game, and I already thought about that. It actually ruins the experience, and it will make the game far too easy. I have liberated both Pagan Min's and Yuma's fortresses with the Buzzsaw in less than 10 minutes as soon as I unlocked and bought it. I'd rather not ruin the single player experience, and have it end up like Borderlands or Dead Island, where players generously drop modded weapons and equipment for new players.