15 years ago it seemed a good idea to build a large number of content sites on a range of topics because these would collect "link juice" and you could either link to your own money sites or sell them to somebody else who wants to do the same. Many of the people who were doing SEO back then are doing paid ads now so it is probably not such a great idea.
Typically you'll need revenue and can sell at a multiplier. Just building a ready made product with no revenue isn't going to pay for the time you spent coding it. An info product might be a better first step to making income in the near term.
Code can be imitated but traction is priceless.
15 years ago it seemed a good idea to build a large number of content sites on a range of topics because these would collect "link juice" and you could either link to your own money sites or sell them to somebody else who wants to do the same. Many of the people who were doing SEO back then are doing paid ads now so it is probably not such a great idea.