An online marketplace, such as eBay, is an example of two-sided markets. In online marketplaces, there are transactions between sellers and buyers and direct competition effects among sellers and among buyers. Previous research on two-sided market has not handle these competition effects and mostly focused on indirect network externalities between sellers and buyers. We analyze the fee structure of an online marketplace considering direct competition effects among sellers and among buyers. We find that when fees for sellers and buyers can be imposed, an auction or fixed-pricing rule can be chosen according to the characteristics of a product traded. But when only fees for sellers can be imposed, an auction can be taken as a trading mechanism solely.