It is common knowledge that authorship of academic and research articles is a major criteria for career progression in many fields, especially in academia, where "publish or perish (POP)" is a very guiding unwritten principle. 

In a daring outing, about three websites based in Russia, Iran, and Latvia have been reported to be serving as domains for brokering academic/research article authorship. How does this work?

The original/corresponding author of the article comes online to advertise articles that have been completely prepared, under review by the publisher, or just accepted prior to publication, and potential authors respond to strike transactional deals that simply involve due payments.

The most sorted offers are articles that are under review or have been accepted (especially in ranked journals). After the settled deals, the original (corresponding) author will notify the publisher about their decision to edit authorship (list of authors), where the new authors are added to the accepted journal.

Following several petitions, top-ranking journal publishers have launched thorough investigations into the operation of these online domains, and many published articles have been retracted based on evidences of authorship sales.

Article retraction under this circumstance is detrimental to the reputation of all authors in the article. This is because the article will not be pulled down by the publisher. However, the article will remain accessible online and tagged as a retracted article, with the reasons for the retraction clearly attached to it. The names of all the authors will also remain tagged in the article, and the full, readable version of the article will never be available. This means the author(s) will not be able to publish the article again in any reputable journal. Also, any author named on such a retracted article will remain in the database, and this may make it difficult for such author(s) to successfully publish even new articles in reputable journals due to integrity issues.

Source
https://retractionwatch.com/2021/09/07/introducing-two-sites-that-claim-to-sell-authorships-on-scientific-papers/