I've been selling on Amazon for the past 5 years and they have only gotten worse. As an example, September-mid-November had bugs and glitches that caused sellers to lose sales (customers literally couldn't check out).
Amazon (like always) denied there was even a problem. Another thing that I don't like is their feedback system. Customers can leave feedback for any reason, but if it doesn't fall into a stock set of reasons, a seller can't ever get it removed.
During the Amazon glitch-fest, an item I cancelled wasn't removed from my listings. A customer ordered it and I had to cancel their order. They then left negative feedback. Amazon wouldn't remove it. If I get enough negatives, my account could get banned.
Customers will also almost always win in any dispute, which makes it very easy for scammers. You also can't block customers from buying.
I also love the fact that amazon charges you to collect sales tax (in addition to having to buy a $40/month account, you need to pay 3%).
The only reason I stay is because by far, they have the most amount of traffic and I get sales as a result.
Luckily, I have other sources of income.