When eBay LINK is first set up it will automatically import all products from Shopify/BigCommerce. If you add products to Shopify/BigCommerce eBay LINK will automatically import those products into eBay LINK – allow 15-30 minutes for them to appear in eBay LINK after adding them to Shopify or BigCommerce. 

If you want to list them on eBay you will then need to map them to an eBay Category.