The short answer
Protect personal data, prove the phone's condition, and avoid payment and shipping scams.
Step by step
Back up the phone and confirm that photos, messages, and authentication access work on the replacement device.
Pay off and unlock the phone when required, then remove SIMs, eSIMs, accounts, wallet cards, and activation locks.
Record the serial number and photograph the powered-on screen, sides, cameras, and any existing damage.
Use a marketplace with documented payment and dispute protection; ignore overpayment, gift-card, and off-platform requests.
Erase the phone only after the backup is verified, then ship with tracking and retain proof of delivery.
Before you finish
Keep screenshots, receipts, chat transcripts, confirmation numbers, and the first bill after a change. Carrier terms and account eligibility can change, so verify the final price and conditions directly before submitting an order.