Motivated by the enormous business success of E-tailers and their distinct business strategies, this paper analyzes the characteristics of dual online channel competition and the fundamental willingness of an E-tailer to open its marketplace to other retailers while at the same time competing with them. We build game theory models to study the dual-channel competition between an incumbent E-tailer and other online retailers under different supply chain structures. Either the manufacturer or authorized third-party retailers can start an online store in the E-tailer’s marketplace. The results show that the transaction fee charged by the platform and the service level provided to customers play significant roles in deciding the marketplace business strategy—the E-tailer faces complicated issues when these two factors fluctuate. A pure strategy of raising the transaction fee may not always be beneficial and a competitor’s superior service level may help to enhance a rival’s sales price. In the expanded research, dual online channel competition with an unauthorized third-party retailer, which is common in the online marketplace, is also examined.