How do I sell backlinks to the SEO industry?

You can sell backlinks in various ways in the SEO industry. No doubts backlinks are one of the top ranking factors for SEO and it is still demandable in the whole SEO industry. But you need to focus on quality backlinks i.e high authority. Low authority backlinks are useless. So if you have a website with high DA then you can sell it easily in various marketplace like Fiverr, UpWork, Freelancer etc. But I think fiverr is the best place to sell backlins.