Build an Online Store (multi-vendor e-commerce website for digital products selling)

We wanna build a multi-vendor e-commerce website for digital products selling. For this website, we have following requirements: 1) This e-commerce website should base on a sophisticated open-source e-commerce platform or the former platform you developed. We do not wanna you develop everything from scratch. We expect you to complete this website development ASAP. 2) This website should have: desktop version + mobile Apps for IOS and Android system. 3) The desktop version should be mobile-friendly, e.g., based on responsive web design (RWD) or adaptive web design (AWD). 4) This website will support multi-language displaying. In the 1st version, we only need English + Mandarin (simplified Chinese). But the website should be easy to be extended to support additional languages later. 5) This website will allow users to create their own account and sell their own digital products online, like Amazon. But we focus on digital products online selling. The online products might be documents, videos, images, .zip files, or any other popular digital format files. 6) The website does not have any product shipment service, instead, the client should be able to download the digital products once they have paid the money. It means: we do not want to see any configuration web pages for different shipping methods in the website backend or frontend. 7) The website should provide text-based online chatting service between the buyers and sellers. The chatting record will be saved locally at the user’s client App or browser. 8) The website should support online payment from the buyers via Paypal and Alipay. In order to protect the buyers, the payment from buyers will first be transferred into the website official account at Paypal or Alipay, not directly to the sellers’ Paypal or Alipay accounts. Once the buyers approved that they are satisfied with the digital products they bought, or they do not have any complaints in a specific period, like a week, the payment from buyers will then be transferred from the website official Paypal or Alipay account into the sellers’ corresponding accounts. We are expecting this payment transferring procedure to be fully automated. 9) The website should support the user to share product web pages into their accounts at following social networking sites: [login to view URL], [login to view URL], [login to view URL], [login to view URL], [login to view URL], [login to view URL], [login to view URL], [login to view URL], chitu.com. It also allows the website administrator to share selected product web pages from the backend into the website official accounts at above social networking sites. 10) The website should support sending a text notification to buyers and sellers via email, like the other e-commerce websites. Moreover, the administrator can send a group email to selected users in the backend. 11) We would like you to integrate an open-source question-and-answer (Q&A) site, similar to [login to view URL], into the e-commerce website platform. It means: if the user has any questions relating to some products selling on the website, they can directly go to the internal Q&A site to ask their questions, and to wait the discussion from the sellers or the other users. This internal Q&A site should support English at least. It would be better if the Q&A site can also support Simplified Chinese. The Q&A site should be easy to support additional languages later. 12) This e-commerce website and its internal Q&A site should be developed for running on cloud servers, providing quick response for users located in China, Western Europe, and North America. You should find which cloud platform is better for our purpose. If you have the technologies and experiences for completing above task, pls let us know: 1) Your solution to above requirements, e.g., which open-source platform you will use, which are the new developing tasks you will do. 2) Your developing time schedule, e.g., how long it will take, including the testing. 3) Your quote.