The phrase "sell one's soul" refers to making a deal or compromising one's own values or beliefs for personal gain. Synonyms for this phrase include "make a deal with the devil," "barter one's integrity," "betray oneself," "go against one's principles," "sacrifice one's values," and "compromise one's morals." All of these phrases convey the idea that a person is giving up something important or sacred in exchange for something they want. Using these synonyms can add variety and depth to your writing, while still conveying the same meaning as "selling one's soul".