I bought my last car on Craigslist. It was quite pleasant and I like the car. Not that there isn't garbage/scams there, but there's garbage on marketplace too (more, to my eye). I make a point to always go to Craigslist first. It still exists, and it's kicking!
OfferUp also has a ton, as does AutoTempest and even good old fashioned AutoTrader. I'm currently shopping for my son.
Facebook seems to have the lowest cost cars, but it also seems to be the least successful way of contacting people. When I bought a car for my sister in April, I contacted (no exaggeration) 70+ sellers and heard back from about ten of them. And yes, I changed the default "Is it still available?" message to something demonstrating urgency e.g. "I have cash and would like to buy." But with persistence, I found her the right car through Marketplace. Really hate the FBook search interface though, it's total garbage.
I spend most of my time looking on CList and OfferUp.