Trust is a main factor of the development of C2C communities. Based on scores rated by buyers and sellers after transactions, computing reputation is a way to build trust in C2C community. This paper proposes an improved reputation computing model (E-Sporas). The results of simulations indicate that E-Sporas has better effects, confronted with fraud behavior, such as reputation slander, reputation collusion. The study gives directions for the development of online reputation system.