After stealthily assembling a NoMad site offering 135,000 buildable square feet, Gary Barnett’s Extell Development is looking to sell the property, sources told The Real Deal. The six-story office building at 30-32 East 29th Street and air rights from nearby properties at 28 and 34 East 29th Street could fetch up to $550 per buildable square foot, or north of $74 million, sources said. The site is located between Madison Avenue and Park Avenue South.