Use this unique identifier to check if the phone has been reported lost or stolen on sites like Stolen Phone Checker. Essential Hands-On Checklist
If you are meeting in person, perform these physical and software checks: Steps after buying a used iPhone - Apple Stack Exchange what to ask when buying a used iphone
Ask for the "Maximum Capacity" found in Settings > Battery > Battery Health . A percentage below 80% generally indicates the battery will need replacement soon. Use this unique identifier to check if the