This paper illustrates the evolution of trading services from the commodity economy to Internet trading platforms from the perspective of non-rivalry using the infra-marginal analysis method and analyzes the monopolistic behavior of platform firms. It is found that the emergence of Internet trading platforms is rooted in the increased non-rivalry of trading services. When producers can freely enter and exit the trading service industry, even if there is only one platform, the decentralized equilibrium is still socially optimal. When there are barriers to entry in the trading service industry, the decentralized equilibrium may result in industry monopoly, a complete monopoly, or an oligopoly of the Internet trading platform market. The non-rival nature of trading services means that the number of platforms does not change with price within a certain interval, so the welfare losses caused by monopoly can be alleviated or eliminated by increasing competitors or by imposing a mandated price. This paper theorizes trading Internet platforms from a new perspective and proposes corresponding governance policies to maintain the environment of competition in the digital economy.