I started this small project for booking tennis courts in 2018. It was free until this year, when I started charging clubs for using my platform. Out of 15 clubs, 14 clubs agreed to pay for my solution. I'm doing some updated, no marketing at all. It's a side project.
Should I sell it? Work on it? Leave it?
It depends on the selling price. And your personal financial situation and the time you can invest in.
It sounds like keeping it for now can mostly provide passive income, which is great. And it also sounds like you could expand it to more paying clubs if you outreach to other clubs. If neither of those are too interesting to you, maybe sell to someone else who is interested. :)