I have bought and sold several iPhones and Apple Watches for myself and family on Swappa over the years. It's sooo easy to use, and when buying I like how many filtering and sorting options they have so you can find the exact device you want at the price you want.
Way less hassle than FB Marketplace because you have a wider selection and don't have to set aside time to meet in person.
When selling, I get way more for my old devices than when doing trade-in.