FB Marketplace is the best, most-used outlet.
What you have to do is price your stuff correctly, you have to say, yes, you have to type this - "I will not answer any "is this available" messages, I will not answer scammers. Scammers need not even respond."
99% of the idiots/scammers will get filtered out through that single message alone. If you put that in your ad, you can 100% ignore the "is this still available" messages. I ignore and block them.
It's fairly easy to filter out the bad from the good if you have a brain, some common sense and street smarts. It's fairly easy to sell stuff. I can tell you that if it doesn't sell within a month, at dirt cheap pricing, nobody really wants it. Right now, I have a used once (like ONE night) Kelty 3-person tent on Marketplace for 6+ weeks. I've gotten two hits on it, they never get back to me after answering their questions. I've got it knocked down to $30. This is every bit $125 tent new. It is nice, clean, high quality.
Most stuff.... I sell within a week. Cars? I clean them up, describe them well, take great pics and price them to where I have 2-4 people fighting over them. I usually sell cars within 4-6 hours.