Most of your money, in the Silent Auction, will be made in those final moments. Anything you cna do to create that sense of impending loss of an item they are bidding on, works to your benefit. Verbal reminders are often best(via auctioneer), but the "town cryer" approach with this sign is great.
A fun idea for your auction. We sell it to someone who keeps it for one year, then returns it to be sold the following year. It's clearly a "gag" gift, but buying it shows a true commitment to the Cause. Sold for $200