Some friends made a sports site for fun. They were a bunch of FB marketers in the early era and just experimented, plus it was a topic they loved ranting on. They did FB ads then funneled the traffic to a WP site.
They hit 4 million pageviews/month in less than half a year. Automated ads didn't work, they paid too little and dropped quality too much.
Selling backlinks became one of the core business. They'd actually write full articles for a client and link to their site. The clients were extremely happy with the results. It's not necessarily the ranking, but the article itself would show high on Google.
Also in my own startup experience, I'd buy backlinks from minor bloggers who were blogging on a similar topic. It was something like $10 for a mention. I don't know if it hurt or helped SEO; but it increased credibility in the community and a lot of the people we interviewed mentioned that they bought it because they saw a familiar name.
This is 2016 data though, so take it with a pinch of salt as things could have changed significantly since then.