He ain’t afraid of these Gotham ghosts. The Merchant’s House Museum is so haunted that its staff actually hired its own ghost hunter — who spilled its bone-chilling secrets to The Post ahead of Halloween. “Manhattan’s Most Haunted House,” at 29 East Fourth St. between Bowery and Lafayette, belonged over a century ago to Seabury Tredwell, a wealthy merchant who lived there with his wife and eight children — and paranormal investigator Dan Sturges says the family never left.