Worth checking how well any option handles scoped access too, since partner integrations down the line usually need their own API permissions rather than one shared login for everything. Stock and pricing sync is another spot where generic setups tend to fall apart once discounts vary by customer, so that logic needs to sit close to the catalog itself. You can check out open source ecommerce here https://medusajs.com/ . It handles cart, checkout, and inventory already, and the same catalog it manages can get pulled straight into a mobile app or handed to partners through its API later on.