The auction Web site known as eBay has become a vast marketplace, bringing together buyers and sellers of all sorts of goods. It has also become a handy laboratory for testing ideas in economics about markets and prices.
In general, auctions ought to serve as an efficient mechanism for setting prices. Consequently, you would expect that similar items would fetch similar prices.
It turns out that on eBay, comparable items may sell for different prices, depending on such factors as whether the sale occurred on a weekend and whether a picture accompanied the item description.