Within the same act, you can leave the crap lying around near the BF merchants. But you need to take it with you from act to act.
Your better off getting rid of it unless you really think you will need it later - odds are you will find better equipment, I know of no key you need to take from act to act and the gear that is part of a set is useless unless it is better than what you have (in which case equip it) since there is no bonus for completing a set.
