No wonder crowds have been a constant at the year-round indoor farmers market that houses dozens of local vendors above the Haymarket T station. The 28,000-square-foot marketplace offers something for everyone: Local residents pick up fresh Siena Farms produce or Stillman meats for home cooking. Tourists flock for the gourmet foodstuffs from Q’s Nuts and the Boston Honey Company. Nearby workers stop in at lunchtime for meals from Bon Me and Beantown Pastrami Company. Throw in hands-on culinary classes, and it’s easy to see why the market will be open daily starting July 18.