Learning to be loyal. A study of the Marseille fish market

We study the wholesale fish market in Marseille. Two of the stylized facts of that market are high loyalty of buyers to sellers, and persistent price dispersion, although the same population of sellers and buyers meets in the same market hall on every day. We build a minimal model of adaptive agents . Sellers decide on quantities to supply, prices to ask, and how to treat l oyal customers. Buyers decide which seller to visit, and which prices to ac cept. Learning takes place through reinforcement. We analyze the emergence of both stylized facts price dispersion and high loyalty. In a coevolutiona ry process, buyers learn to become loyal as sellers learn to offer higher u tility to loyal buyers, while these sellers, in turn, learn to offer higher utility to loyal buyers as they happen to realize higher gross revenues fr om loyal buyers.