When you're selling your home, there are lots of defects you're required to disclose to a prospective buyer, but the presence of a ghost isn't one of them. That's the law in Massachusetts.
But if a buyer walks through your home and gets a creepy feeling, and actually asks whether the house is haunted, you and your agent are duty bound to respond truthfully.
While you may not be able prove there's a ghost wandering your halls at night, you may be aware that someone was a victim of a violent crime or committed suicide in the house, but again, you don't have to disclose that unless you or your agent are asked.
If you'd like to visit some spooky places during this Halloween season, there are many venues throughout New England where you can do that, just for fun. Boston Magazine has put together a great list of the "Best Haunted Houses," while Ghost Buster extraordinaire Jeff Belanger will be delivering several "Supernatural Evening" lectures at local libraries and colleges to regale listeners with true tales and legends of unexplained hauntings--believe them or not.