"Be sold on" is a colloquial expression that means to believe in something or to be convinced by it. There are a few synonyms that can be used to convey the same meaning. For instance, "convinced" is an excellent option; it suggests that one is persuaded or has been won over by an idea or perspective. "Firmly persuaded" is also a fitting one; it reflects a sense of certainty and assurance regarding one's beliefs. Additionally, "all in" is a more informal option; it implies that one is fully committed or dedicated to a particular idea or course of action. In conclusion, utilizing synonyms can be a useful way to add variety and precision to one's language.