Just to add another 2 cents, I've bought and sold on CL many times over the years. Never met a scammer or killer. Met some pretty nice and interesting people though. A deal typically involves at least a 10 minute convo about the item in question and swapping stories about the activity in which the item is used e.g. the substance abuse counselor who bought a jeep from me to transport his clients to job interviews and retreats... the drywall contractor who bought an autofeed screwgun who for 45 minutes discussed construction in general and the current contract he was working on. And numerous people who just talked about life in general.
The worst experience I've had on CL is the no-shows.