I sell all my old phones on eBay. I would recommend eBay or Craigslist.
Just be careful buying a used phone for Verizon. If it has a bad ESN they will not activate it for any reason. eBay is good for this reason. If you buy a phone and it turns out it has a bad ESN and the listing didn't say it had a bad ESN you would be covered under PayPal buyer protection and get your money back.
"Is life so dear, or peace so sweet, as to be purchased at the price of chains and slavery? Forbid it, Almighty God! I know not what course others may take; but as for me, give me liberty or give me death!"
-Patrick Henry, March 23, 1775