I had a Merch by Amazon account in which I invested many many hours of work, been gaining much less than the work invested, until the holiday season, when the hard work was about to bounce and show fruit.
Guess what, exactly on the 30th of November 2018, Amazon closed my account after the best selling month, they obviously stated as a reason to terminate my account, Content Policy(The ambiguous, unclear, fragile and highly corruptible Amazon Content Policy) where literally anyone can be hunted down for infringing copyrights, in fact, there are many just copyrighting daily use quotes or expressions in such a way that it's not safe to even describe a product in an intelligible manner and remain in the copyright free terrain.
Amazon obviously stayed with all the winnings of the highest selling month, printing and sending "copyright infringing" clothing Made by Amazon without any issues. Just a coincidence? Google it and you'll see how many have been brought down for absurd reasons around the highest selling season of the year with Amazon taking ALL THE PROFIT for themselves.
We "signed" the terms hoping to have some extra income and ended up losing, Amazon wins. If you consider getting in, I would recommend you just find a more independent way of doing business far from Amazon, or at least don't expect too much of it. Thousands of work hours can go down the drain literally when Amazon deliberately decides to flush you.