This article analyses the influence of trust on the functioning of a fish market, where agents can choose between bidding or exchanging through bilateral transactions. Even if it is well accepted in economy that trust plays an important role in transactions, its definition and its measurement stay, as far as we know, very elusive. Starting from the empirical analysis of the Boulogne-sur-Mer fish market, a market where people have the choice between trading through auctions or bilateral exchanges, we show how the social networks structure differ between the auction market and the bilateral one . We then propose a measurement of trust, based on the dynamics of agents encounters. We bring into the light that, when the transaction links on the auction market reflects the economic constraints of the partners, the relationships on the bilateral market depend on something more. Clearly, the bilateral transactions result from economics and non economics determinants.