One or more proxy bidders is associated with a rule which determines a time to place a bid. The proxy bidder places a bid at the determined time, instead of immediately after the bidder's prior bid is exceeded. Such rules may be selected for all proxy bidders to encourage a desired auction behavior. By controlling the placement of bids based on rules associated with the desired auction behavior, bids may be placed in a manner that seems natural and competitive to bidders in the auction when, in fact, they are placed in accordance with the selected rules. The desired auction behavior may be selected statically for an auction or dynamically during the auction. Historical auction data maybe analyzed to characterize the behavior of an auction, such as the average period of time between bids and the average increment between bids. Rules may be generated to correspond to a behavior so as to provide the same average time and increment between bids.