I’ve been doing it for around 4 years now. Growth has been slow initially because you need a lot of listings to be noticed - the bigger sellers all have over 1000.
I knew I wanted to do something with public domain art prints, so I researched other sellers to find the types of things that were popular. Lots of places like the Library of Congress, NYPL and the Rijksmuseum have open access policies, so I regularly hunt through their collections to find interesting stuff. The problem is always finding something in the public domain that’s also very high resolution.
For a long time I produced the pieces myself as buying top quality paper and a decent printer worked out cheaper than using a print company - and I quite enjoyed it.
However I’ve decided now that the convenience is worth the extra cost.
I’ve also automated creating listing images by using Cloudinary and a few shell scripts. This has been a massive time-saver.
As for print companies, it’s a balance between cost, quality and price. The big ones like Printful or Gooten are cheaper and do everything from t-shirts to phone cases, but have a limited range of paper and sizes. Specialist fine art places have a huge range of paper and produce better prints but are much more expensive.