Is becoming an Amazon seller profitable? Is it worth paying for a course?

You probably have things backwards. You start by having products you want to sell, and then if you think Amazon may be the right way to sell it, then you try to find out how Amazon works, and that's where courses may be useful (or not).

There's no magic "I become an Amazon seller and I suddenly make money" formula. You need to find products to sell. You need to find people to buy them from. You need to buy them (i.e. pay them). You need to have the stuff shipped to Amazon. If it comes from abroad, you need to handle taxes and duties and be sure they comply with local regulations. You have to create your seller account. You have to create listings for all products, with pictures, description, all sorts of details. You need to select prices, and possibly adjust them constantly. You have to handle support requests. You probably need to do some advertising (either on Amazon or elsewhere) to bring customers in. And of course you have all sort of tax-related stuff on top of that, including probably setting up a company and a bit of accounting.

It's just a business like any other. A combination of a good idea, good execution, and hard work could make you rich. Or could swallow all your money if you're not prepared or try to sell the wrong product, or sell it at the wrong price or forgot about many of the small details that can make or break a business.

Amazon just simplifies a lot of things for you: it lets you relatively easily be present on one of the biggest stores in the world (if not the biggest), they will handle payments, they can handle shipping out to customers faster than you could ever do it yourself. But you'll be just one of millions of sellers, selling a few of the millions of products on the store.

The other earlier answers are correct about having a business selling stuff on Amazon.

But also consider this:

The true value of such a course is if (and only if, IMO!) it offers detailed instructions on how to obey the rules of the Amazon markeplace. Amazon enforces all of its rules whether you know them or not - ignorance is not an excuse! - and there is very little ability for a seller on their marketplace to appeal, even less chance for it to be a successful appeal. Then, if you're kicked off or suspended temporarily for breaking their rules and you try again even with a different name, or account you'll get kicked off permanently with no appeal.

If you already know what you want to source and sell, and if you've already worked out the economics and want to give it a go, then the Amazon marketplace can be profitable for you. Then, if the course teaches the Amazon-given rules so you use it correctly, the course might be valuable.

(My claims about rules, Amazon's enforcement of them, and the penalties - possibly permanent - for breaking them, can be easily confirmed by anyone browsing the Amazon seller forums. (You may have to create a seller account just to look at the official Amazon seller forums - I forget - but there are plenty of unofficial ones around too for you to look at. Actually, it would be a good idea to find some of these and browse them for awhile just to get an idea of what you're getting into.)