Your backpack only has 4 slots. At the end of a game, if you were victorious, you can collect all the item drops into your item stash and/or backpack. When selling, you see both the backpack and the item stash. The item stash is not accessible during battles, only the backpack is.
So if you want all the items from the battlefield, make sure you win! Otherwise, you can run from battle with your backpack only.