Is is legal to use a quote from an unknown author to sell in a greeting card?

Yes. Even a quote from a known author on a greeting card wouldn't be copyright infringement.

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.

If you require further advice on this topic, I practice in Georgia and can be contacted through my avvo page.