a homogeneous market for honest participants. That community ratings (of goods) do in fact strongly influence buyer behaviour is shown empirically in [@SDW06].

Existing reputation systems are fragile, in that they can easily be distorted or abused even within the frame of laws governing them. ‘Attacks’ of this kind, though not proper attacks in the sense of information security, threaten the integrity — with respect to its purpose — of the informational content stored in the system. Dellarocas [@Dellarocas00] classifies unfair behaviour into the categories 1. *Ballot stuffing*: A seller colludes with a group of buyers in order to be given unfairly high ratings. 2. *Bad-mouthing*: Sellers and buyers collude to rate other sellers unfairly low to drive them out of the market. 3. *Negative discrimination*: Sellers provide good services only to a small, restricted group of buyers. 4. *Positive discrimination*: Sellers provide exceptionally good service to some buyers to improve their ratings. A situation of controlled anonymity in which the market place knows the identity of participants and keeps track of all transactions and ratings, but conceals the identity of buyers and sellers, is identified as essential to avoid unfair behaviour. For instance, anonymity is an effective protection against 