It doesn't exist yet. But it should (maybe).
My idea is to have a website that displays listings of real estate that has plummeted in value because a notably violent death occurred on that property. I think there's a good amount of people who would be willing to pay less for a property such as this, but they lack the foresight to consider this rare price-reducing factor. So, in that sense, I'm targeting an untapped market.
It's very possible that there are serious road bumps, but I think anytime you can figure out a way you might be able to target a specific group of people to sell to effectively, it's worth considering.
It would require some regular research though. I'd have to find articles or crime reports that verify that an incident did in fact occur on the property in question.