If you want to swap items easily with your other characters, then the easiest way to do it in single player is to use Claptrap’s Super Secret Stash.
You’ll find Claptrap’s Super Secret Stash on the northwestern side of town. You won’t be able to access it until he calls you for a short side quest to find his stash.
You only get four slots in the stash, but it’s better than nothing.
