Good thing about merch marketplaces, (if you choose the right one) you make an image design, description, title, and tags, and which merch it will appear on, (maybe some formatting) and then thats it. Set and forget. If someone buys your $24.99 you generally get $3-5. So like i say with a trend, its easy to hit $50 out of the gate quick, if you choose something smart and intersting that people flock too. You don't stop at one design. Make 30 your first stop, then 300, then 3k, if you want to really dominate.
Its not like etsy, where you are shipping things physical. The marketplace that you upload your design to does all of that. Even handles returns. You design, upload, name, and collect cash. The 4 easy actions! lol
Whats good also is that they have their own payment gateway for each and or sometimes paypal.