An auction in which the winner is the person with the lowest unique bid. For example, if one person bids five dollars and two people bid three dollars, the item will go to the person who bid five dollars. However, if one person bids three dollars and nobody else bids three dollars or anything lower, the item goes to the person who bid three dollars. Recently, a man in Korea won a $44,000 BMW for $5 in this kind of auction on the Korean auction site

Oddly, this is the only case of this kind of auction I could find with a google search for "upside-down auction." This term can also be used to mean reverse auction.

According to Achromatic, the term "hammer down auction" is used in Australia to describe the same process.