In this paper, we present a framework for evaluating the security of trust and reputation systems for electronic marketplaces populated with buying and selling agents. Our proposed framework offers a method for researchers to understand the security of their systems, and to provide precise guarantees of the degree of provable security that these systems offer. We demonstrate the viability of our proposed framework by presenting a specific monetarybased trust system known as Trunits, along with an analysis that shows that Trunits provides a guaranteed level of security for buyers.