I second this - I would buy this today if I could.
Did I make a huge and expensive mistake in deciding to buy this house? Here's $50. Be honest.
No. Your house is not going to kill you, your kids, or your pets, within 99.6% certainty.
Awesome. Best $50 I ever spent.
But the sort of person who would buy your testing service is also the sort of person that does not buy houses that typically have such hazards in them. You are mostly going to be validating subjective emotional impressions with objective empirical evidence. It's a very powerful feeling to have someone else prove without a doubt that your gut feeling was correct.
For a small minority, you will actually be identifying real household dangers, so it's a win-win business.