You can always sell back to Apple themselves. They have a buy back program, not great, but it is an option. Personally, I go Craiglist or Ebay. That's been my goto. I know there's other sites like Reverb and Bonanza, but I'm not familiar with those. Oh yes, Facebook Marketplace is the new comer. A very annoying user interface. I have a hard time just finding the Marketplace, let alone the settings for items I'm selling. And the amount of seedy messages I get from potential buyers is beyond annoying. From one person, I had difficulty telling if he was a scammer or legit buyer. Even after he called me and tried to meetup in person, I was getting messages asking me to send him a 5 digit code, which made me wonder if someone had hacked into his account because he had sounded very sincere on the phone.