Teespring is a company that allows you to make a custom line of tshirts and hoodies.
Check out the site below:
https://teespring.com/
In theory, you could create a tshirt line and market it on Amazon as well to generate passive income when someone buys the shirt. There is no requirement to buy the shirts and sell them yourself.
Is this an idea you have considered in the past? Seems legit.