In this paper, we provide new proofs of existence and uniqueness of a Stackelberg market equilibrium for a multiple leader-follower noncooperative oligopoly model in which heterogeneous firms compete on quantities. To this end, we consider a two-stage game of complete and perfect information in which many leaders interact strategically with many followers. The Stackelberg market equilibrium constitutes a pure strategy subgame perfect Nash equilibrium of this game. Existence and uniqueness are more difficult to handle in this framework, insofar as the presence of several leaders and followers displays a richer set of strategic interactions. Then, to prove existence, we notably provide a new procedure to determine (the conditions under which) the optimal behavior of the followers (may be written) as functions of the sole strategy profile of the leaders. Some examples outline our procedure and discuss our assumptions