The previous owner has to disable the Activation Lock and remove the Apple Watch from the list of the registered devices.
Have a look at this article:
About Activation Lock on your Apple Watch - Apple Support
The previous owner can also do that online:
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Turn off Activation Lock on the web
If you can't access your iPhone or Apple Watch, or if your Apple Watch isn't paired with your iPhone and you need to turn off Activation Lock:
- Go to iCloud.com and sign in with your Apple ID.
- From the Quick Access Menu in the top right, select Find My. You might be required to sign in.
- Select All Devices, then click your Apple Watch.
- Click Erase Apple Watch. Select Next until the device is erased.
- Click the remove button next to your Apple Watch.
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copied from: About Activation Lock on your Apple Watch - Apple Support