There were lots of companies that were in the rental business (of books). They had agreements with Amazon and built their business based on this agreement. Amazon completely pulled out of the rental business one day last year and all of these companies went under nearly over night.
Amazon did the same thing to me around 15 years ago. I used to sell items on Amazon (non-books) and built a fairly large business. Amazon slowly figured out how to out-compete me and had eventually cheaper items in every category. Many of these categories had no Amazon product until I started selling them.
Amazon eventually banned my seller account with no reason given. I had near 100% positive feedback and only had a couple of negative stars, which were resolved on my end by allowing the customer to keep the product and I 100% refunded. When my account was banned, I had almost $50,000 in my account that they wouldn't let me touch for almost 6 months. It put me out of business. I was luckily able to get all of my money back and didn't go into debt.
It took over a decade to finally get my seller account back. No reason given.
My take away is that no matter how tempting, never build your business based on someone else's platform. You can start out this way, but the plan to move away from it should be there from the beginning.
Isn't Amazon being sued by SEC for this exact thing?