It's a good idea. I'm sure many of us are good at digital investigative work.

My only concern would be, speaking generally, the risks and your credibility.

Ie: I track down a bail jumper online only to find out the higheree just wanted to spy on his ex-girlfriend.

With that worry aside If I were to be contracted legally I would only feel comfortable accepting payment upon competition. So you would have to use an escrow service.

Nice idea.