This paper explores the rationale of price-taking and price-making behaviours in the context of Walrasian and Cournotian pure exchange economies. Beside the influence of the number of agents, we underline the role of the structure of preferences in the definition and in the working of market power. Through three equilibrium variations of the same basic economy, we obtain several results about price manipulation, about asymptotic identifications for large economies and for degenerate preferences, and about welfare comparisons. Perfect competition does not only correspond to the case of large economies, but may also concern economies where fundamental market powers are more or less equivalent.