Bidders coordinate their behavior to increase their surplus by eliminating competition among each other,which is called collusion. In general, auctioneers will try to forbid the collusion because collusion may decrease the benefit of the auctioneers. But, auctioneers in a new style of online auction, group-buying auction, should encourage the bidders to collude because the collusion will also potentially increase the auctioneers' benefit.