I created an ebook a while back on how to make money with Airbnb (based on my experience as a superhost), but I did zero marketing and ended up making about 3 sales total...
A few years back I made a course on Udemy. I figured this would solve my marketing issue. It took me 6 months and was a real slog but managed to create a course about Neo4j. I don't make a tonne of money from that, but managed to make a best selling course, and on a good month it might bring in $1k USD.
A couple years ago I did Sam Ovens consulting accelerator which basically teaches you to reach out to ppl with problems and get them on a sales call then sell them a $2k solution to said problem. I followed this and managed to sell a handful of people on helping them land their first dev job. People's problems were varied, I realise in hindsight that I could have been more educated around coaching but I did help all those people land their first job. I played with pricing a bit, selling at various prices ($2.5k, $750, $1.5k). I feel like if you pushed long and hard enough on this niche perhaps it could be successful, but to me it always felt a bit off for various reasons.
Now, I'm creating a website builder. I'd say it's not a project for the light hearted - it pains me a bit to say that it's been a year so far. I actually have something that can build a website and did have it up online, but took it down to avoid server costs while building it out.
I've learnt a lot on my journey and I find it fun (in a massochistic sort of way), but still very much a work in progress.
Happy to share if anyone has questions :)