There are some things that are better left unsaid. They are the types of things that are so big that everyone can assume that unless you say otherwise, that they aren't true. Like saying "I don't have herpes" right before having sex with someone is very unusual. It's highly specific and apropos of nothing. Don't even bring it up unless you actually have it. That's the same thing with haunted houses. If you say "this house isn't haunted," guess what? That house is now haunted - at least in my brain it is.
No one drives by houses for sale and wonders if they are haunted, but once you put a sign up about it you better believe that's what they think. Even if you're trying to fight off a bad reputation of the house being haunted, that sign isn't going to do you any favors.