What is a Paid Link?
A paid link is a backlink you receive in exchange for payment. The payment could be in the form of money or something else of an equivalent value.
Paid links are against Google guidelines and are considered a black hat SEO technique. Google will demote the rankings or even deindex sites discovered to engage in this practice.
That said, we should clarify that Google permits the sale of links as advertisements. But they must be clearly labeled with the sponsored attribute.
But paid links violate these guidelines and are usually set to dofollow.