There is nothing wrong with courses. Just because some are bad doesn't mean they are all bad.
Amazon FBA is super competitive now for a few reasons:
1. CPC price are at an all time high, and you won't survive if you ignore PPC to get traffic and it will eat your margins big time.
2. Chinese factories are now becoming smarter, and instead of selling to you, there are now going direct and removing the middle man, so you won't be able to compete based price.
Not to say, it is not doable it is just harder in my opinion.
My advise is focus on good branding and high margin products that way you won't suffer with cash flow issues and restocking inventory. Good luck in your journey