Hi
Follow the steps here to remotely erase your Apple Watch and remove Activation Lock from it:
- About Activation Lock on your Apple Watch - Apple Support
- Unpair your Apple Watch and iPhone - Apple Support
It may help if the buyer next follows these steps:
On their iPhone: close the Watch app (double-press on the Home button, then swipe up on the Watch app preview).
Restart both their iPhone and Apple Watch, turning both off together, then restarting their iPhone first:
Re-try pairing and setting up the Apple Watch:
If the Apple Watch then remains locked by Activation Lock, then:
Either accompany the buyer and the Apple Watch to a Genius Bar appointment, taking along your original proof of purchase for the watch.
Or arrange for the Apple Watch to be returned to you, re-pair it with your iPhone and then unpair it via the Watch app to erase it and remove Activation Lock before returning it to the buyer.
Or contact Apple Support and explain that you have been unable to remove Activation Lock from an Apple Watch that you have sold:
- Contact - Official Apple Support
- Apple Support (@AppleSupport) | Twitter
- Genius Bar Reservation and Apple Support Options - Apple
Note that you should never disclose your Apple ID and password to the buyer, even if you change your password again immediately afterwards:
Security and your Apple ID - Apple Support
More information: