E-commerce, despite its many advantages, is still not living up to its full potential. To mitigate this problem, several trust management systems in the form of online ratings have been developed. The basic idea is that the system maintains reputation or trust-level of the buyers and sellers, and makes them available to other interested parties to make decisions to engage in a transaction. One of the reasons is the lack of trust that users have in Internet-enabled exchange of goods. The quality of the reputation value depends on the quality of the feedback provided by transacting partners. However, feedback is susceptible to various threats that render the trust management systems untrustworthy. This paper examines the vulnerabilities of feedback-related issues of trust management systems and suggests a mechanism to mitigate these threats.