The phrase "sells down river" refers to betraying or abandoning someone for personal gain. Some antonyms for this phrase could include "loyal," "dedicated," "trustworthy," "dependable," "reliable," or "faithful." These words all suggest that someone is committed to a cause or person and would not allow personal interests to compromise their principles. Other antonyms for "sells down river" might include "uphold," "protect," "safeguard," or "defend." These words imply a willingness to defend or stand up for someone or something, even in the face of personal sacrifice or difficulty. Overall, antonyms for "sells down river" are characterized by integrity, devotion, and a sense of moral responsibility.