Your key objects are things like "seller" and "bidder" (both of which are "parties"), "items" (things that you are auctioning), and "bids" (an offer by a "bidder" to pay a certain amount for a certain item, if this bid wins).  An "auction" associates a "seller", and an "item" with a collection of "bids".
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An auction can also be reformulated in terms of a regular order system, or vice versa (an order is simply an auction where the bid and ask prices are fixed and always agree; or an auction is simply a way of negotiating the final fixed price of an order). --LairdNelson
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See ProvenSystemMetaphors
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CategoryMetaphor