In this paper, we will concentrate on the potential of reputation for optimising electronic transactions, liable to fraud and cheating. No surprise, electronic auctions have been the first arena for the implementation of computerised reputation systems. Economists expect that the probability of cooperation is positively affected by online reputation reporting systems (e.g., eBay), but electronic communities present several opportunities and forms of misbehaving also at the level of reputation. In eBay, rated transactions are about 50% of the total and positive ratings exceed negative ones. This evidence is interpreted in terms of a social cognitive model of reputation, allowing formulating testable hypotheses about the different performances of different reputational systems.