I find it worse than Craigslist, especially now that Craigslist charges a $5 fee to post an ad. Keeps out the riff raff. 

Facebook Marketplace has a clunky interface and also does not make it easy to tell if a car has a rebuilt title or not. A lot shadier people dealing there than Craigslist, where I have bought/sold countless cars. Most people who reply on the marketplace ads are "I will let my son know", "selling for a friend" type people. In other words, time wasters.

They are on Craigslist too but not nearly as bad. Also remember that it is very easy for the marketplace crazy people to find your Facebook page and post all kinds of junk.

It’s no better or worse than Craig’s, in my opinion.  Just make sure it has a clean title and have an independent, honest mechanic check it out first.

Could be a good place to find cars, just like craigslist. But make sure to take all precautions when buying a used car and use you common sence and you should be fine. I personally prefer craigslist and places like marketplace and offerup have more fake/scam listings but still as long as you act smart why not.

as long as you can check it out, and test drive it before paying, why not

Same as Craigslist.  If you are buying a car on there, have a mechanic check it out.  You never know when someone might try to scam you.