Most link building services either scrape sites with “write for us” pages and farm them or just resell posts from massive private blog networks that are masquerading as guest posts.
Additionally if you do Google searches like this or like this you can find lists of thousands of sites that publish links for payment.
Google aren’t stupid and it would take an engineer less than an hour to write code to detect and devalue all of those links so we have blacklisted them as well to make sure you only get high quality links that move the needle.
In short, the blacklist (along with our quality control review) guarantees your repeat custom.
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