While numerous studies have addressed the issue of price formation in first-sale fish markets, little is known about the purchasing decisions of buyers. Using exhaustive data on 11.7 million transactions made in 2021 and 2022 by 1955 buyers in France, we provide an unsupervised classification that reveals the heterogeneous behavior of buyers in terms of quantity purchased, average price paid, number of markets visited, and type of transaction. The results of a hierarchical agglomeration cluster analysis led to eight buyer groups. We show that there are a small number of very active buyers, both at auction and over-the-counter, who purchase daily in large quantities and at many different markets, and whose main activity is wholesale. We find that the price-quantity elasticity values for first sales are very low, but the price paid by large buyers is even less sensitive to quantity than that observed for other buyer groups.