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

I used eBay to sell a pair of JBL L110's - you just state in your listing that it is for pickup, no delivery, and cash when picked up. That way you dont have to deal with PayPal fees but you do get the exposure of ebays reach.

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. 

because of high price of repair if sent to B&W consider sending to a tech --or simply bringing it in.. This highly depends on where you live and be careful of businesses that over charge.   Frankly,  resale for parts best bet is ebay because of such a huge buyer base--but you will spend listing fees and final fee is at least 10% of final cost ..  I would check for local tech .  If you live in southern Cali I can recommend a very good shop and while he has gone up a bit in price he is very reasonable to the extent that I ship to him from northern Cali --