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.

Question:

Explain what key attribute the artwork must have for the English bidding system to raise the expected level of the winning bid, and why.