Use a Legitimate Channel
Anyone can sell their iPhone, but you can’t trust just anyone, especially when it comes to data security and phone quality. That’s why we highly suggest avoiding classifieds like Craigslist at first, where it’s more or less impossible to guarantee quality without traveling somewhere and checking the phone out in person (and even then things can go wrong).
Instead, try some of the major used phone sites that coordinate more professional sales. Check out Swappa, Gazelle, and Glyde to see what they offer and their average prices. These are popular sites with some built-in protections for buyers, making the process much easier. Glyde is essentially the closest thing to buying a refurbished phone that you can find, as they run rigorous tests on all phones before reselling them – but you can probably find better prices on the other sites.