One downside to selling your book on Amazon is that you don’t have the readers/buyers’ information. You don’t know who bought your book, and you can’t communicate with them. This isn't an issue exclusive only to them if you sell through other distributors. (Where Should I Publish My Book?)
Amazon keeps the emails of everyone who shops with them, and they are not sharing. It would be irresponsible of them if they did. The only access you have directly with buyers and readers is the one you create. Amazon does allow you to create an author page. Part of your funnel can start there.
It depends.
If you’ve ever heard of self or indie (independent) publishing, then you probably know about Kindle Direct Publishing (KDP). Amazon’s platform is a massive, global marketplace and Kindle Unlimited, their e book exclusive program. Between the two of them, they are the first and second biggest bookstores in the world.
You can publish a book with them in a day.
