Um, are you sure this wasn't a former Indian burial ground?
Real Estate agent Jake Palmer admitted over email that the sign was too attract attention to the house, and that there wasn't a real danger of it being haunted. However, that doesn't mean the sign isn't accurate - indeed, the house is NOT haunted. At least to Palmer's best knowledge.
If you're interested, this house in St. John, New Brunswick, Canada, is under $200K, and nice enough inside that you might be ok with the fact it's only probably ghost-free.