There seem to be some apps on the Shopify store which allow you to manage and sell through Amazon & Ebay directly through Shopify
Codisto seems like the most famous apps for this. You can download it here https://apps.shopify.com/codisto-linq
You can also create an Amazon Sales Channel on Shopify to keep track of your products across Shopify and amazon. You can refer to this article for more details https://help.shopify.com/en/manual/sell-online/amazon-sales-channel
Let me know if this helps!