rel=sponsored
Google recommends sponsored for links that are advertisements, paid placements, sponsorships, or otherwise part of a commercial arrangement.
rel=nofollow
Nofollow can be used when a publisher does not want to associate the site with, or pass ranking credit to, a destination. Google also accepts nofollow for paid links.
rel=ugc
UGC is intended for links inside user-generated content such as forum posts and comments.
Attributes do not change the commercial relationship
A disclosure and a rel attribute serve different purposes. Publishers should be transparent with readers and use technical attributes appropriately.
How this site handles merchant links
Backlink Buyer marks its commercial merchant CTAs with rel="sponsored nofollow". We do not hide the commercial nature of those links.
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.