The use of software-based agents to act on behalf of human bidders for dynamic participation in multiple simultaneous online auctions is disclosed. The software-based agents may reside on computer systems or on any type of stationary or mobile terminal. On the basis of bidding-related information from a bidder, a software agent selects a plurality of auctions to place bids in. Upon being outbid, the agent determines whether to place an additional bid in a further auction. The agent can make such a determination on the basis of maximising profitability or surplus.