Used apple watch

Activation Lock is designed to prevent anyone else from using an Apple Watch if it's ever lost or stolen.

About Activation Lock on your Apple Watch - Apple Support

Activation Lock is an anti-theft feature that cannot be bypassed. Only the previous owner can remove the lock by using their Apple ID & password. Apple will not help you find the previous owner. If the previous owner cannot or will not remove the lock, then the watch is useless to you or anyone else. Try to get a refund (through eBay) or dispose of the watch properly. In the future, see the 'Before you buy a watch from a third-party' section in:

What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

Here is how the original owner can unlock the Activation Lock:

• With a web browser, go to: https://www.iCloud.com and sign in with his Apple ID. Open Find My iPhone and select All Devices (at the top). Then click his Apple Watch, select 'Erase Apple Watch', and select Next until the device is erased. Finally click "X" next to remove the Apple Watch.