I'm a little surprised. Do people think that sharing that info with a private investigator would be unethical?

Answer is, that its not clear cut. What is the difference between a private investigator (working for a single party) and the government investigator (working for the city, state or federal level)?

I would think, that advertising in the local press or, having a sign outside the window to the effect that you were able to sell personal and assumed to be private information about the people who lived in the street would provoke an unpleasant reaction.

Doing something purely for your own benefit that you know will cause significant distress to others is sociopathy I think. Perhaps 'unethical' is the wrong word. 'antisocial' definitely covers it though it might not be strong enough.

I used the private investigator here as a middleman, a third party. Just like law enforcement or a data broker would be. My concern (in this argument) is less that of the ethics of each single act, but rather the consequences of setting up such a service. You don't know what you are enabling.