The main rules for Merch by Amazon product listings are as follows:
You must have the rights to any designs you load. That means you should stick with either using original designs you created yourself or those you hired someone to create and are 100% certain you own the rights to.
You have to make sure all designs meet Amazon’s content policies. That means no illegal content, offensive content, or content that directly solicits reviews or suggests the sale will have proceeds going to charity.