Method 1 (Best): Ask the seller to turn off Find My iPhone in front of you. Then the device is ready for your Apple ID.
Method 2 (Limited): Use Apple’s iCloud Activation Lock Status page (part of iCloud.com). You need the device’s IMEI/MEID (not serial number) for this.
Actually available:
Red flag: If a used device is offered with “iCloud unlocked” cheap, it’s probably stolen or a scam.
AppleCare+ extends coverage beyond 1 year. To check: apple serial number check apple
If you’re buying a used device with claimed AppleCare+, ask for proof of transfer (AppleCare+ is transferable, but you must call Apple to reassign).
Possible reasons:
A serial number check helps you: