Merch by Amazon is an incredible program for making money. Great for designers, Or anyone who wants an extra side hustle. Unfortunately, It's easy to lose your account.

CHECK TRADEMARKS

This is a huge one. So many people don't know the difference between copyright and trademarks. It's very important to know. I enjoy using
TMHUNT
Or use Trademarkia
There is also the government website.
I've noticed that even short sentences can be trademarked which can make things difficult. I normally don't worry about single word trademarks unless they are made up words, Such as Google, Steemit Etc.

Don't Reference Anything

When you are posting a shirt, Think about where it came from. If you are trying to reference a sports team, Movie, Song, etc.
DON'T Post it! Sounds simple right?
If your shirt design wouldn't exist without "X." Avoid it.

Reverse Image Search

For those of you who hire others to make your designs, Always do reverse image searches on Google to ensure that your designer is not stealing graphics they shouldn't be. Often even those "free" sites where it's allowed can get you in trouble as Amazon is cracking down on copycats. Be extra careful with people on Fiverr.
I've noticed a lot of designers on there will sometimes do things they shouldn't. Don't be afraid to pay a little more for quality, Or hire someone who will work long term (They will often give you better pricing.) Upwork is a great site for that.

Conclusion

It doesn't take much to lose your account, And sometimes it can be hard to get an account in the first place.