The online auction site eBay allows buyers to enter a beginning bid on an item and then automatically increment their bids upward to some maximum bid level. Sellers are allowed to enter a minimum price that they will accept for their items, called a reserve price, below which the item will not be sold. Use the concepts of producer and consumer surplus to explain why this setup is attractive to buyers and sellers. Why does eBay promise that it will not reveal the seller's reserve price or the buyer's maximum bid to other users?