Yeah....I've happened upon on some shady deals in the past from FB and Craigslist, guns and other things (dirtbikes, boats, vehicles) after a while of digging and asking enough questions, if it's a con / stolen they'll disappear in a hurry.
In the thousands of posts I've browsed through this year there's only been one instance of a stolen firearm being posted. On the other hand there's been hundreds of posts about stolen firearms.
IMHO if someone's posting under their own name, pictures and descriptions of firearms for sale, willing to give you cell numbers, meet and make the deal they're probably legit.