In a successive vertical oligopoly, a set of "sellers" produce some input to be transformed into a final product by a set of "buyers". On this two-sided market, a firm's profit increases with the number of firms of the other type and decreases with the number of firms of its own type. We examine the emergence or the entry of a new marketplace sponsored by a profit-maximizing intermediary who targets buyers and sellers in sequential way by setting membership fees (or subsidies).