Glad it turned out well for the victims.
I have done a lot of buying and selling on CL, only had one prospective buyer make me nervous. I'm expecting one guy, and three show up, the smallest having 4 inches and 60-80 lbs on me (and I ain't that little at 5'10"/185# and fit). I made sure to keep at least 8-10' between me and them while they were looking over the vehicle, and deliberately made the grip of my Witness Compact 10mm visible in my waistband. Nothing came of it. I think they were legitimate, but they low balled me, I declined, and that was that.
Pay attention to your spidey senses when conducting private transactions with strangers. If it feels wrong, don't ignore that instinct.