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".
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
See
ProvenSystemMetaphors
CategoryMetaphor