The phrase "is in business" can be replaced with a variety of different synonyms depending on the context. For instance, instead of saying "John is in the retail business," one could say "John operates a retail store." Alternatively, "John is in the business of selling products" would also work. Other synonyms might include "John is a merchant," "John runs a business," "John is an entrepreneur," or "John is a proprietor." The choice of synonym will depend on the tone and formality of the sentence, as well as the particular industry or field of business being discussed.