Buying links to manipulate search rankings is link spam; legitimate advertising or sponsorship needs a clearly qualified relationship.
Paid links include cash, goods, services, advertorial placement and other consideration. Payment is not inherently prohibited; the risk arises when placement is intended to pass ranking credit or conceal its commercial relationship.
Practical implementation
For advertising or sponsorships, document purpose, placement and disclosure, then use the relationship value recommended by guidance such as rel=sponsored. Keep editorial link decisions separate from procurement. For historic placements, inventory them before removal so remediation stays traceable.
What to verify
Review the live placement, disclosure, destination and rel attribute in rendered HTML. Keep a register of paid relationships and recheck pages after publishing systems rewrite attributes. Follow any manual-action instructions directly.
Common mistakes
Do not disguise a commercial link with a generic anchor or URL anchor. Wording does not change intent.