House Network (opens new window) in the UK is offering a ghost-free home certification (opens new window) service, and they point to a report that says that:

Houses can sell for up to 20% less than market value, and twice as hard to sell, if they're thought to be haunted. I wonder how much less it can be when they're not haunted?

Audits are performed by Ghostfinder Paranormal Society (opens new window):

Ghostfinder Paranormal Society are an experienced group of professional ghost hunters and paranormal experts that operate across most parts of the United Kingdom investigating reports of hauntings, poltergeists, residual energy and all forms of paranormal or supernatural phenomena.

Questions are asked and video/EVP/temperature/EMF readings are taken.

Customers have to pay extra, between £100 and £500.

"House Network cannot be held responsible for the results of the investigations, effect to the property's price or failure to sell."