Amazon doesn’t buy the third party products to resell to the client, nor does it pass delivery and production contracts with volume orders and quality checks etc.

It’s a completely different business that has a different relationship with its sellers. In particular Amazon guarantees nothing and the seller is fully responsible for delivering the goods and satisfying the client. Going through Amazon’s warehouses is still a different case, but if I’m not mistaken it’s just a shipping agreement and nothing more.

In specific cases Amazon might actually buy the goods from the seller first, but it’s the exception more than the rule.