The best way, to create your products on Shopify & eBay marketplace, is using 3rd party integration app. The Integration app can list your Shopify products on the eBay marketplace directly.
If you use the eBay integration app, you only need to create products in one place SHOPIFY with all products' details (Title, description, Image, Price, etc) and for the central management of inventory, price, and order.
the eBay integration app will upload your products directly from Shopify Store with the eBay marketplace. Shopify can be used as a central management for Inventory & price that will be sent automatically on eBay marketplace and when you will get an order on eBay marketplace the integration app will sync/create them with Shopify store. From the Shopify store, you can ship the order by updating the tracking details and that shipping details will be automatically updated on the eBay marketplace.
In this way, you can have single & central management.