Gazelle will pay you with a check, PayPal, or Amazon gift card after you fill out a questionnaire with similar questions to Apple's: if the phone works, what condition is it in, and a some other basic information.
After answering, you'll get an estimate and Gazelle will send you a box for shipping. If your phone matches your answer, you should be paid within 10 days. For my iPhone XR, Gazelle estimates that I would receive $400.
You can check out Gazelle's service right here.