As a book seller:
1) Customers aren't mine. I'm not allowed to send any of the people that buy from me a link to my website or collect any of their data. This means that this income channel can be taken away at the whim of Amazon.
2) Amazon regularly uses your sell data to undercut you and put you out of business. I regularly would sell rare books with no other listings. As soon as I had any sales, Amazon would popup with their own listing at half price. My sales would go down when this happened.
3) Strange things would go on with their listings. I was in business for 5 years. I saw patterns and know when my sales would be up and down.
None of these rules applied on Amazon. I would be going strong for weeks at a time. One day, I would have no sales and this might last for a month. From 100 sales/day to 0 just seems to be a bit funny to me.
Customer support would of course deny everything. However, after running my own tests, I could see that from certain IP addresses and on some Amazon servers, I would get random errors on checkout of my own listings.
Amazon eventually banned me. I had near 100% feedback and 1 complaint from a user that I sold them the wrong book. Before the user complained, I even paid to have it shipped back to me. This wasn't good enough. The customer wanted to keep the >$200 book and not send it back. I nearly went broke because I couldn't access my money for 3 months.
It took me a year to get my business back to where it was, but I am now in a better place (with many sales channels).
Amazon is an evil company and anyone that does business with them is risking everything for short-term gains. It's the destroyer of small businesses.