Hi
You are describing Activation Lock, which prevents unauthorised use of Apple Watch if it is lost or stolen.
To be able to pair the watch to your own iPhone, you would need the previous owner to either:
- Enter their Apple ID and password on your device, or;
- Turn the feature off via https://www.icloud.com/ by signing in with their Apple ID > Settings > My Devices > Click on Apple Watch > Click on the "X" > click on Remove.
If CeX cannot arrange for the previous owner to turn off Activation Lock, you should return the watch to CeX for a refund or exchange.
You will not otherwise be able to use the watch (there is no way around this security feature).
More information:
About Activation Lock on your Apple Watch - Apple Support