Backlinks buy consists of buying external links on websites belonging usually to the same themes than the website to promote and having a good PageRank or a good reputation on search engines.
The goal of buying backlinks is to increase the website popularity, the number and quality of external links being taken into account by search engines for sites ranking on SERP.
Backlinks buy can be done directly or by going through a backlinks selling platform. Concerning backlinks buying, the buyer makes sure it relates to text links and choices anchor texts.
Search engines disapprove and sanction buying backlinks but this practice is nearly undetectable while done privately because nothing distinguishes a bought backlink from a natural link.
Selling websites can nevertheless sometimes be detected when they pass by backlinks selling platforms or when the practice is public.
JC Penney has been punished by Google in early 2011 for having bought many backlinks.
An example of backlinks trader :