C2C e-marketplace is a growing online business segment where users enable to sell and buy products online directly. This research finds out the influencing factors to buy and sell products in C2C e-marketplaces of Thai users by separated analyzing for buying side and selling side. Collecting 220 online survey responses for buyers and another 220 for sellers from targeting on C2C using Thai people. Adapting factor analysis technique to find out the influencing factors and verify significance by regression analysis. The results indicate there are 3 main influencing factors; 1-various and unique products, 2-website’s reputation and 3-convenience of usage. While selling side also has 3 main factors; 1-amount of users, 2-convenience to sell and 3-attractiveness to buyer.