A method and a system generate a reputation value for a user in a network-based community. A processor-implemented transaction data collector module collects transaction data of users of a network-based community. A processor-implemented transaction graph generator module generates a transaction graph based on the collected transaction data. The transaction graph has a transaction relationship between two users, and a weight corresponding to the transaction relationship. The weight is representative of a mutually reinforcing relationship between two users. A processor-implemented reputation generator module generates a reputation value for a user from the transaction graph.