Is this legal ????
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Not sure about in Missouri. Here in Louisiana, the case could be made that accepting payment to assist in locating a person could conceivably be considered a private investigation, and while the person(s) making the offer (presuming they were not doing so with the intent of committing a crime)would be ok, the person accepting the money and furnishing the information would be acting as an unlicensed private investigator. It might be a stretch, but the case could be made. Even then, they'd probably subject to a cease and desist order from the PI licensing board.
I sense its skating on some pretty thin ice. there is what appears to be an implied threat “therein”