I've sold a couple of things (DVDs, a kids' bike, and a toy box) thanks to one of those groups. It was less hassle than putting the items on Craigslist and then having to put up with calls and emails from scammers and BS artists.
I did have one lady who went psycho on me because she believed she was "first" to claim my item but I ended up selling it to someone else. It turned out that the person I sold it to responded under my post on the page, but the other person sent me a private FB message about it...which went into my Facebook spam folder so I never saw it. Oh well, ya live & learn. The message she sent me after the fact was so abusive that I probably wouldn't have wanted to deal with her anyway!