I think "sold" probably doesn't mean what you think it means here. Have you looked at the projects? Nearly all of them are just statics lists of things. Like "a list of all country flags" or "a list of modern inventions"

Maybe it was some SEO related sale with the domain and bit of ad revenue that someone bought for $100, which probably isn't the definition of selling a side project that most users on HN have. In fact, looking at the domains, the domains were probably worth more than the content on those sites.

That's why I originally flagged this post because I thought it was a bit misleading. I didn't think most people here would consider simple lists of things as side projects.