Where would be a good place to buy& sell used audio equipment?

Try FaceBook Marketplace for local sale. I used to use Craigslist a lot with some success. Marketplace seems to be what everyone is using these days and I have sold things much quicker. 
One thing if you dont have facebook they wont give you marketplace right away till you use facebook a while "puke". So i gave up and just post on my wife's account.