Problem
Ebay has the most popular web site for Internet auctions, auctioning everything from Pez candy holders to fine artwork. Ebay's auction system can be characterized as an English system with a seller reservation price, and a time limit - but it may be more than that.
Consider a piece of artwork. At the time he or she posts an image of the artwork on the Ebay site, the seller can set both a reservation price and the opening and closing times for the auction. Ebay members may then bid on the artwork in a manner similar to an English auction. That is posted bids are observable to all.
Task
Explain what key attribute the artwork must have for the English bidding system to raise the expected level of the winning bid, and why.