The sale is authorized to pay back unpaid rent. If the self-storage unit sells the items, they're obligated to record how much it sells for so they can subtract that money from the debt.
If you just throw everything from several units into one sale, you can't track how much income went to which debtor.
That said, you don't HAVE to sell it all. Auctions dominate because you sell all of it to the buyer, and disposal of useless stuff is their responsibility. But you could sell what you could sell in a rummage sale, donate and dispose of the rest, as long as you give the original owner credit for the proceeds.
If in doubt, talk to a lawyer.