My experience on FB running groups
Start your own group, add pretty photos and info - make it interesting for folks to join and share.
Make it a private group
Ask 3x specific questions for entry that folks have to write a thoughtful sensible answer and not just "yes"
You can control the folks that come in, you write your own rules, you can ban the bastards
Don't just deny the join, ban.
The majority of shitty accounts join overnight because of regional timezones.
It's nice to be "inclusive" but all your sales will be from USA, maybe Canada.
Australia is full of gold folks, but the rules are sticky, knives get confiscated by customs, shipping is HUGE and slow. Nothing but problems. UK and euro same
If Aus UK and Euros are serious, let them find or set up a USA shipping proxy
Ban the second and third world profiles, they won't be paying customers,
They will be spammers, or pakistan makers stealing your photos and then offering to sell you your own knives.
Eighty percent will post something for sale against your rules, ask how they can emigrate, ask to be hired, ask for you to sell them some piece of equipment and ship a 4 ton machine to Africa, India or Asia
Ban most of the women and all of the pretty Asians. They are not customers, they are spammers.
There's an exception for wives, men using their wife's account and widows needing help selling their husband's stuff.
Ban all T shirt selling scum
Ban all business profiles and accept only personal profiles with legitimate sounding names, friends that are from where they are from and so on.
Don't have the name of your city in the group, you will get a hundred brain deads that think it's a local buy and sell garage sale group.