Have signup sheets at the auction table. One sheet per item. Bidders can sign up for any one or more of the items.
At some unknown, mystery time it is announced that the high bidder on each sheet is the winner. These can be random times selected by someone not near the bid tables.

Another variation is to have the same type of signup sheet but announce that the auction will close at a given time. It's up to a bidder to make sure he has placed his high bid on any of the items he wants.

Have fun collecting!!