Backpack getting a little too heavy?  Starting your own legendary shop?  Well you're going to need somewhere to store your rainy day legendaries, can't be carrying all that precious cargo around on your main, now can you?  Well make a mule!  Just create a new character, load a co-op game with both your main character and your new level 1 character and give the level 1 the items you want to store. It's as simple as that.  If you want a really epic bank, you might want to get some more slots ...