I've said it before, I'll say it again...I buy a couple grand/month in parts on FB for quite some time. I generally stick to the vehicle specific groups though. My rules are 1) never try to buy something you need right then 2) buy every deal you can make a buck on, whether you need it or not 3) If there's one for sale, there will be another...they're facebook parts, not a Gutenberg Bible. 4) Offer a fair price, when they reject, that's fine...50% of the time, they'll be back. Then lower your offer price because you've probably already bought the part you need (see #3), and don't need an extra (see #2).
That said...I've refused to list anything for sale in a public forum or FB for a decade, because of all the Jimmy's in the world. If I see someone needs something I have, I'll offer it. Funny thing is, most still want it for free.