Debit cards are fine.
And we really have no clue about what is in the units. We do not cut the locks on them until the auction starts, while everyone that came to bid is standing right there watching. (Some storage facilities have been said to "bait" their units, meaning they cut the lock before the auction, look through the unit, and put the good items in front, in hopes that the unit will be sold for more...cutting the locks with everyone there prevents this)