AirPods Max serial number and model number

My dad gave me his AirPods Max and told me it was replaced before because it was defective. Now, I'm planning on selling it but upon searching the serial number, it says, "this device has been replaced." I don't want to sell it if it's fake because it's unfair. The model number is also different from the box but it is the same for the device itself and when it is connected to a device.