With the advancement in the information age, people are using electronic media more frequently for commercial transactions. Online auction is a prototypical example. In online auctions, bidders or agents can easily participate into many different transactions simultaneously and the number of bidders participating in a given transaction is not bounded. Owing to such benefits, distinct features emerge compared with the traditional auctions, which are reviewed here. There form a number of bidders who are responsible for a significant fraction of the total bidding activities due to the online characteristics. We show that they exert strong influence on the final prices in distinct auctions. This domination of online auctions by such a unusually active minority may be a generic feature of all online mercantile processes. On the other hand, the bidding process in the auction systems is described by using a master equation with the transition probability determined with empirical data. We show that the bidding at the last moment is a rational and effective strategy to win in an eBay auction. Finally, the bidding pattern emerging from the interactions between individual bidders or items is analyzed in the perspective of the graph theory.