Borderlands: share/swap guns with players in co-op?
Solution 1:
Yes, you can share pretty much anything in your inventory except your cash, which is automatically split evenly between players when you pick it up anyway. Loot is not assigned ingame at all.
Note: If you decide to swap items with a friend ingame, make sure you hit a save point before you exit your game if you're not the host, or else it will not be saved in your inventory. If your friend then does hit a save point and the game saves that they do not have the item anymore, it'll be lost in the void and you can't get it back.