I've joined a number of vintage Facebook groups over the last 12 months. 75% of which Ive left.
The ones I can recommend are:
Adam's Echo base- a UK trading group
Ross Barr's Empire Strikes Back group
And Todd's Group.
There are some terrible group out there full of drama and an endless supply one post wonders asking for valuations on ep1 merch.
However the groups I've mentioned are run well with rules that are for the most part stuck to. I've done some great trades and buys. Got my Yoda, vader,
Luke Bespin and Chewbacca ESB MOC's through these groups and had no problems at all and made a few friends along the way.
Once you been on there for a while you'll quickly see who the trustworthy guys are. I've had no problems at all.
On a side note I deleted over 100 "friends" on fb. Just have 40 or so close friends and family now plus all the groups and webpages I follow. It's more of Star Wars news feed now than anything else