If you had a phone replaced by Apple under warranty, the old serial number is dead. Apple issues a new serial number for the replacement device. Ask the seller for the original repair receipt.
In the vast ecosystem of Apple products—from the iPhone in your pocket to the MacBook on your desk—every device tells a story. That story is encoded in a seemingly random string of letters and numbers: the serial number .
Remember the golden rule: A scammer can Photoshop a valid warranty date. You must go to https://checkcoverage.apple.com and type the number in yourself.