A haunted house stigma may be proven to affect a buyer through people refusing to visit them in their home or workers refusing to do work on the property. It may seem trivial to someone with no faith in ghosts, goblins and the like, but it is very important to some people that their home be free of all suspicion of a haunting. Thus, while disclosure may make it hard to sell your home, it may also affect whether the buyers decide to sue you for not telling them about something very important to them.
If you decide not to tell the buyer about the death, she could end up suing you for misrepresentation, in addition to dragging your reputation through the mud. Word of mouth carries a great deal of weight in our industry, so it's important to be honest and responsible with both parties of the transaction.