The most important part of this project is that the website needs to work with 3 existing eBay stores.
When a new product is listed in the website, it needs to automatically list it on the eBay stores as well. When an existing product is edited in the website, it must also be revised in the eBay stores, and so on. When a buyer sends a message to one of the stores, the message needs to be received on the website as well (a specific section for each store), so the Admin of the website can respond to all messages from the Backend of the website. Also, all the sales and any other notifications need to be shown in the Backend of the website, in a specific section for each store. There must also be an option to connect another eBay store to the website.
Each product needs to be connected to the correspondent products in the eBay stores, so the product is disabled/edited, it will also end/revise accordingly in all the eBay stores.
When a product is added in the Catalog the fields must be similar to eBay, with slight changes.