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An auction is the process of buying and selling things by offering them up for bid, taking bids, and then selling the item to the highest bidder. In economic theory an auction is a method for determining the value of a commodity that has an undetermined or variable price. In some cases, there is a minimum or reserve price; if the bidding does not reach the minimum, there is no sale (but the person who puts the item up for auction still owes a fee to the auctioneer). In the context of auctions, a bid is an offered price.

Auctions are publicly seen in several contexts: in the antique, where besides being an opportunity for trade they also serve as social occasions and entertainment; in the sale of collectibles such as stamps, coins, classic cars, and fine art; in thoroughbred horseracing, where yearling horses are commonly auctioned off; and in legal contexts where forced auctions occur, as when one's farm or house is sold at auction on the courthouse steps.

Much more economically important, however, are auctions in which the bidders are businesses or corporations. Examples of this type of auction include:

Internet auctions, dominated by the wildly successful eBay, have become very popular.

The world two largest auction houses are Christie's and Sotheby's. The world's largest online auction site is EBay.

Auction catalogs are frequently printed and distributed before auctions of rare and/or collectible items; these catalogs may be very elaborate works, with considerable details about the items being auctioned.

Types of Auctions

Some commonly used types of auctions are:

If more than one identical item is sold, there are two possible generalizations of the second-price auction. In a uniform-price auction, all of the winning bidders pay the price submitted by the highest non-winning bidder. Bidders will not typically bid their true value in a uniform-price auction with multiple units. In a Vickrey auction, the pricing rule is more complicated, but preserves the property that bidders will bid their true valuation.

Bidders in the traditional Dutch auction and sealed first-price auction will tend to underbid what they believe the item is truly worth in hopes of getting the item for less. This behavior is known as bid shading. These two auctions are also theoretically equivalent, but in practice Dutch auctions will produce less revenue than sealed first-price auctions (one of the important results of Experimental economics)

Work in the theory of auctions contributed to Vickrey's 1996 Nobel Prize in Economics

See Also