and information goods are increasingly occupied by self-organising communities. These market places exhibit the characteristics of the so-called long tail economy [@longTail]. That is, the classical asymmetry between suppliers and consumers is lifted. Buyers and sellers are often even in numbers and may change their roles dynamically. Virtual, or physical, goods are offered in large numbers and diversity and with potentially small demand for each single one. Matchmaking and orientation of buyers is difficult in a long tail economy, long term relationships are hard to build, and trust between trade partners must be established somehow [@Bakos1998].

A common approach is to let market players themselves provide the necessary guidance. This is mostly embodied in reputation systems by which buyers and sellers rate each other and the goods sold, or recommendation systems[, [i.e.]{},]{} programs which attempt to predict items that a user may be interested in, given some information about the user’s profile. Reputation systems, according to Paul Resnick *et al.* [@Resnick2000] “seek to establish the shadow of the future \[the expectation of reciprocity or retaliation in future interactions, cf. [@Axelrod1984]\] to each transaction by creating an expectation that other people will look back on it”. The goal is to establish 