Wow, you've sold a lot of sites. How does that work?
I'd argue there is some kind of magic here.
Solve a problem and people will probably find your site.
Otherwise, if you're solving a specific problem in a niche, start hanging out wherever those people hang out, participate in the community and share your solution if it's applicable to the discussion. Make sure not to spam/self-promote a lot though, as it comes off as really tacky in most places.
this is part of why many now pay creators to start new accounts to post viral organic content rather than to their existing followers.
when you find a video format that works for your product, you can create many accounts (or pay people to do so) and keep posting the winning formula