In standard economic theory, mechanisms like Adam Smith's "invisible hand" or the Walrasian auctioneer balance aggregate demand and supply and match individuals such that the market clears. Usually, some kind of price adjustment process is assumed without specifying how the implied transactions are organized. In real markets, price adjustment and the matching of buyers and sellers involve considerable exchange of information. Past experience plays an important role in partner selection and in deciding whether a suggested transaction is accepted or not. This implies a process of learning about trading partners and opportunities. The model suggested here, explains how this learning leads to market organization (loyality) or a lack thereof (searching). A decentralized market of a perishable good is considered, where past experience governs the choice of trading partner. Depending on how important past payoffs are and how long the memory is, and depending on the number of players in the market, buyers decide to search or to be loyal. The transition from searching to loyality is very abrupt and resembles phase transitions known from statistical physics. Simulations and empirical evidence from the Marseille wholesale fish market confirm the co-existence of the two behavioral patterns of buyers and the importance of past experience of their behavior.
