I'll give you my amazon horror story... I'll readily admit I'm biased on this because I've had a really bad experience selling with them, but research around and you'll find this sort of event happens all the time.
First off, it's easy to sell on there and you can instantly get a huge number of shoppers and sales, more so than any other sales channel that I have ever seen, and I've used many. It's actually quite amazing how fast you can get substantial sales for virtually any product.
Here's our story. Amazon initially contacted us to sell our products on their site. It is a fairly involved decision for an established site because their fees are very high, 10 - 20%, which does include all fees including payment processing fees. When you're going from 2 - 2.5% overhead cost to 10 - 20%, margin compression is a gross understatement unless you're operating on some massive profit margin. We sold on there for about 18 months, while maintaining a 99% positive feedback rating. Without warning, Amazon shut our account down due to policy violations. Apparently some of our products were prohibited on Amazon.com's site, disregarding the fact that they came to us to sell on their site. We were given vague instruction on how to appeal the decision, which was never overturned. Was never able to talk to a single person via email or phone, just a contact support form, which we never even got a response to. Our dedicated account rep went completely silent. The whole situation was really upsetting considering our Amazon account manager personally helped us with the data conversions and they knew full and well every product that we were selling. When you have a customer that is selling well into 6 figures per year on your platform with perfect history, not even allowing them to talk or even email to someone, it's just insane.
Another to consider. This didn't happen to us, but I've talked to multiple merchants that this has happened to that are customers and some I have met at various web related conferences. Merchant starts selling their products on Amazon and Amazon isn't currently selling those same products. If you look in the seller agreement there is a lot of detailed clauses about conversion and shopper data and that it belongs to Amazon and they can use it however they want. Well, they will use it however they want. If the sales and conversion data that you generate meets certain guidelines they will go find distributors and start selling your products. It's very difficult to compete head to head directly on Amazon's platform because they will end up with a lower cost than you, they offer free 2 day shipping for prime, and they aren't paying the 10-20% fee per sale.
So yes, you can build a ton of sales quickly selling on amazon. If you have very good cost on your products it's possible to be competitive on Amazon. However, in the end your efforts may be benefiting Amazon just as much or more than yourself. And at the end of the day Amazon doesn't care if you go away and would be far happier just selling your products themselves. If you think about it, there's fundamentally a huge conflict of interest selling the same products as their merchants. Lastly, if you build a successful business on Amazon, it's really scary for me to think about what happened to us, for an Amazon only business. They would literally have to close their doors without even someone they can get an email response from. Just unacceptable.