The verb transact is almost always used with the word "business." To transact business is to conduct it, as when a shopkeeper sells groceries to a customer.
All kinds of business owners and workers transact business, just by following their usual routines of selling, buying, investing, trading, or anything else they do to carry out their work. Shoppers also participate in transacting business — a purchase can also be called a transaction. The Latin root word, transactus, means "to drive through or accomplish."