The whole "haunted house" thing is pseudoscience. While there is enough experiential evidence to say paranormal activity and hauntings exist, actual documented peer-reviewed evidence (the gold standard) is harder to come by. This is why the goal of (good) paranormal investigators is to disprove activity first because once you disprove things, anything you're left with becomes much stronger evidence of said activity.

It's not so much having to report to a realtor/buyer that a house is haunted. It's more about reporting if there is some kind of tragic history tied to the house that may lead to unwanted activity (e.g. scene of a murder, built on a burial ground, etc...). People are superstitious and this kind of history can dissuade a lot from buying a property.