1. A back-end system that could act as a Middle ware between "Vendors websites" and "Retailers websites"
2. The system itself will be a Marketplace website on the front-end
3. An API that will connect "Vendors website" and Fetch products to the system (Multi vendor system)
4. An API that will Distribute the products taken from "Vendors" amongst other websites I would call "Retailers"
An Order Management system, Inventory system, and a Wallet
The only 2 companies I have heard of that could do something like this are:
1-Airship: they have automated all the processes of dispatching, logistics, customer support, credit analyses, order fulfillment, etc on the backend. In the front end you just built your own e commerce website. It is plug & play and that easy.
They built the order management, inventory system and dispatch on the back of a white label e-commerce site. They are focus on grocery stores so don't know how much it will fit your specific needs.
(Disclosure I know both CEO's and have invested interests
Two great sites are http://www.shopify.com/ or http://www.squarespace.com/. The both have content management systems that are build and managed by the companies, have integrations with Google Analytics and best of all they have the whole billing flow/system taken care of, which for a commerce site is one of the most important things to get right.
They provide base themes or allow you (with a little elbow grease to learn their respective templating languages) to completely control the front end.
If this is for clients you get the added bonus directing them to the Shopify/SquareSpace customer service for more backend technical questions about payments etc.
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