Important - how marketplace sync really works: Amazon SP-API, Flipkart Seller API etc. require server-side OAuth and secret keys - they cannot run from a public web page. This hub manages your listings, orders and settlements in your own database and maps your products to each channel. To make sync live, deploy one Cloud Function connector per marketplace (keys server-side); this UI then reads/writes the synced data. Everything below is real CRUD on mp_listings, mp_orders, mp_settlements.