The link-spam definition
Google says link spam includes creating links to or from a site primarily to manipulate search rankings. Its examples explicitly include exchanging money, goods, or services for links or posts containing links.
Advertising and sponsorship
Google also says buying and selling links is a normal part of the web economy for advertising and sponsorship. Such links should be qualified with rel="nofollow" or rel="sponsored".
Why intent matters
The same visual link can have different policy implications depending on whether it is a disclosed sponsorship that blocks ranking credit or a paid placement sold specifically to influence rankings.
Affiliate links
Google’s spam policy notes that affiliate links can be used throughout a page when treated appropriately. On Backlink Buyer, merchant CTAs are marked rel="sponsored nofollow".
Use primary sources
Provider blogs and SEO discussions can help explain practices, but current Google documentation should be the baseline for claims about Google’s own policies.
Important policy context
Google's current spam policies identify buying or selling links for ranking purposes as link spam. Google also says paid advertising or sponsorship links can be legitimate when appropriately qualified with rel="nofollow" or rel="sponsored". Read Google's spam policies.