Reputation systems are an important factor for building trust in virtual communities. In the article we introduce reputation module for Quality of Protection Modelling Language which allows to represent the reputation system as a part of the protocol, where all operations and communication steps can be consistently modelled. Owing to the proposed approach the reputation systems can be formally specified and computer security impact can be considered as a factor of the metrics in the reputation systems. Finally, we model and analyse the case study of the eBay reputation system with modification which will refer to the computer security impact.