Online auctions are becoming an increasingly important channel for electronic commerce. There exist more than 150 online auction sites on the Internet. It is difficult for users to attend, monitor, and bid at multiple auction sites simultaneously. In this paper, we propose BiddingBot which is a multi agent system that supports users in attending, monitoring, and bidding in multiple auctions. BiddingBot monitors prices of goods in several online auction sites to get reasonable market prices of goods, and uses a new cooperative bidding mechanism to effectively bid in auctions.