I recently watched YouTube videos that showed people doing Print on Demand and making any where between $100 and $5000 within 3 months of starting it. What I see consistently is the fact that these people advertise their shop. Whether it is through paid advertising on Etsy, or through running Facebook ads if they have a shopify account connected to their Print on Demand account. A few videos showed people lost more than they made, and it really depended on the product they were selling. All of these Youtubers did their research though. They literally focused on selling products that met an emotional or physical need. For example, health supplements, dieting supplements, self-help books, etc. They touch on a pain point, and to be frank, its has been the same thing since the 2000's when I first got into internet marketing. It has not changed.