Abstract: This paper develops a duopoly ecommerce platforms competition model with vertical product differentiation between platforms and horizontal product differentiation among sellers within the same platform （Salop，1979），based on the framework provided by Mussa & Rosen （1978） The model is specified through a fivestage dynamic game involving two ecommerce platforms，sellers，and consumers In the first stage of the game，the platforms simultaneously set the access fee for sellers，which would be the only source of profits for platforms；in the second stage，a sufficient number of potential sellers decide whether to enter the platform，with competition ensuring that sellers earn zero expected profit；in the third stage，consumers with different quality preference types choose one of the two platforms as their shopping destination；in the fourth stage，sellers，after evaluating the average intensity of consumer quality preferences on the platform，set the prices of their products；in the final stage，consumers select a seller on the platform that maximizes their net utility from the purchase Before analyzing platform competition，we first solve for the equilibrium under platform monopoly according to the timing above There exists a unique pure strategy Perfect Bayesian Equilibrium with symmetric seller pricing for the dynamic game within certain parameter ranges，in both platform monopoly and duopoly
Under the monopoly equilibrium，it can be shown that as product quality rises，the number of brands and the total consumer base on the platform decrease However，the increase in quality ultimately leads to an improvement in consumer welfare The key findings regarding platform competition are as follows （1）An increase in platform substitutability enhances both interplatform and intraplatform competition However，a rise in horizontal substitutability of products within platforms mitigates intraplatform competition without impacting interplatform competition （2）As “high quality” rises，the welfare gains for consumers on the highquality platform from the increase in quality itself outweigh the welfare losses for all consumers due to reduced interplatform competitionMoreover，as  “low quality” rises，the increase in total consumer surplus exceeds the reduction in platform profits caused by intensified competition Enhancing either “low quality” or “high quality” can increase total consumer surplus and social welfare However，given the same level of improvement，raising “low quality” has a greater impact on total consumer surplus，while increasing “high quality” results in a more significant boost to social welfare

Key words: E-commerce platform, interplatform competition, intra-platform competition, vertical product differentiation, Salop circle model

摘要： 1 China Institute of Regulation Research，Zhejiang University of Finance and Economics/The New Type Key Think Tank of Zhejiang Province “China Research Institute of Regulation and Public Policy”;
2 School of Economics，Shandong University

关键词: 电商平台, 平台间竞争, 平台内竞争, 纵向产品差异, Salop环形城市模型