You haven't answered my question. How do nodes verify that a person making a transaction actually owns the external unit? And how do you prevent that person from spending that external unit elsewhere outside of the network? Since it's external, and the transaction only happens on the network, the unit itself hasn't actually changed hands. The original owner still possesses it, and the new owner has no way of physically obtaining it. Explain how this problem can be solved without requiring a central depository.