The communication within Internet auction systems proceeds as a rule under the situation in which users are not in physical contact nor they do not know anything of each other. They have therefore to rely on reputation mechanisms implemented within these online systems. Such mechanisms help to create a trustworthy environment on the basis of additional attributes associated with users and their roles. The trustworthy environment in online auction systems (trust of the system itself and trust among users of this virtual world) is the essential element for these systems functioning. This paper introduces a trust model based on reputation while it takes into account possible fraudulent behavior of users in online auctions as contextual information. The reputation is calculated from user’s evaluations (feedback) following performed transactions. Information about possible fraudulent behavior is additional information determining the reliability of the user’s reputation in our trust model. Reputation and fraudulent behavior are expressed in a form of belief functions and the resulting user’s trustfulness is calculated. The case study shows that the proposed approach is valid and may be applicable in real online auctions.