How do I sell the print publication rights for my book?

I don’t have any experience there; the way that I would go about it would be to find an agent in your niche that has contacts that would be interested in printing your book, including in overseas markets.

If you are making a lot of digital sales, you could try approaching publishing companies yourself as you’d have some data behind you, but unless you go with smaller independent companies you probably won’t get very far as the big players all use agents as a minimum cost of entry to vet new projects.

The agents you’d need to speak with would all be specific to your niche; you could try looking in the Writer’s Market annual books for the addresses and contact information for book publishers and agents; I used to purchase that book every year back in the 90s, and it was a pretty quality publication back in the day so I assume that it still is.