Which P2P shopping portals enable users to have a synced estore simultaneously on portal and it’s Facebook app so that when user edits estore on portal then the same changes would reflect on the equivalent Facebook page or app?
Again, there is no such thing as P2P shopping. P2P means Peer to Peer and is a form of file sharing. C2C means customers to customer, B2B means business to business. Neither is at all relevant to the question, of course.
To answer: Shopify has Facebook apps which sync with your store. http://apps.shopify.com/facebook-store