I just wondered how any ordinary writer makes money through Amazon. My supplier, YPD, give me 100% of my book price. If anyone buys my book via Amazon I lose 20% of the price, but if I stock my books with Amazon their commission is 60% which leaves me making a loss of about a £1 per book on the cost of producing each book.
Also if I do not stock with Amazon, anyone who seeks out my book and doesn't use Amazon UK, they tell potential customers that my book is out of print and they do not know when it will be available.
Also of course I have to pay tax on my meagre profit whereas Amazon famously pay no tax on the 60% they take.
With my reader's hat on, my first choice for buying new books is direct from the publisher or author if that's an option. Failing that, I check independent bookshops. If they haven't got the book on their shelves, can they order it for me? The only books I (reluctantly) buy via Amazon are by authors who have self-published with them so have no other outlet.
With my writer's hat on, I try to spread the message to readers that Amazon isn't the only place that sells books!
Ahh, what's an ordinary writer? Amazon has its pros and cons. It sounds as if you have to give up front costs to your supplier. If you publish through Amazon, you have no up front costs, which is wonderful for those who want to see their book in print, but are too poor to pay up front costs. But of course, it doesn't get your book in front of readers, that is where the costs come in. Maybe an "ordinary writer" with an agent and publisher to promote the work will have a hard time making money out of Amazon, because, of course, they're paying for using their system as well as having paid costs to said agents etc, and competing for that commission to promote the book.
And of course, you can't blame Amazon for their "out of print" notice, for to them, it IS out of print and unavailable. After all, they aren't there to promote competing services.
I think the answer to your question is, only wealthy, and or established writers get to make money through Amazon or any other outlet.