This paper discusses the effectiveness of sharing information concerning the reputations of buyers and sellers making online transactions in a consumer-to-consumer (C2C) market. We developed a computer simulation model that describes online transactions with a reputation management system that shares information concerning the reputations of consumers. The model takes an agent-based approach in which agents' actions are based on the iterated prisoner's dilemma. No model exists to analyze C2C markets even though there are many case studies concerning the effectiveness of sharing reputation information among participants in a market. The simulation results revealed that a positive reputation system can be more effective than a negative reputation system for an online transaction, even though the negative one can work for a traditional transaction. The result should be an important consideration when designing practical reputation management systems for online transactions.