The phrase "sold one on" means to convince or persuade someone to do or believe something. Conversely, antonyms for this phrase would include words or phrases that describe the opposite action of convincing someone. These could include terms like "dissuade," "discourage," or "convince otherwise." Additionally, antonyms could include phrases like "lose confidence in," "reject," or "disbelieve." Ultimately, the concept of antonyms for "sold one on" would encompass any action, belief, or sentiment that counters the notion of persuasion or convincing, including terms like "disagree," "resist," or "ignore.