Platform sharing across manufacturers has become common practice in the automobile industry. Although platform sharing reduces the degree of product differentiation, manufacturers can reduce their procurement costs by taking advantage of the commonality of components. We investigate this trade-off through analyzing a model that incorporates manufacturer–supplier relationships into a differentiated duopoly model, and find an interesting inverse relationship between the advantage of platform sharing and manufacturers’ costs to communicate with their potential suppliers. We also explore welfare consequences of the Internet trading exchanges by considering an extension that allows the manufacturers to jointly establish a business-to-business electronic marketplace.