This paper examines current approaches used in reputation management and defines five basic metrics for the evaluation of reputation of Trusted Third Party (TTP) hosts employed in mobile agent-based e-commerce applications. By taking these metrics into account, a new distributed reputation management scheme is proposed for quantifying and managing the reputation of the TTP-hosts based on a transaction-based feedback system. The scheme exhibits several interesting features not seen in previous work: firstly, it offers better efficiency and robustness for an application that employs it, and secondly, it credits/penalizes a TTP-host according to its transactional behavior, the transaction value and the reputation of the source of feedback. In this way, TTP-hosts may be deterred from misbehaving.