Bought a used Apple Watch and it has activation lock

Click on this link to read "About Activation Lock on your Apple Watch" --> About Activation Lock on your Apple Watch - Apple Support

Your resolution possibilities are to either get the account working again or ask Apple to reset the Watch (this will require you have convincing proof of ownership such as the original receipt of purchase from Apple)

For the first, you're operating under layers of security that Apple has around devices and then the accounts required to unlock devices. From what you say you are at this stage:

If a message says your Apple ID is locked or disabled "for security reasons"", the document If your Apple ID is locked or disabled - Apple Support directs you to reset your password as outlined in this article --> If you forgot your Apple ID password - Apple Support

That then involves a messy process since you probably don't have access to the account's trusted devices.

If you use two-factor authentication and can’t sign in or reset your password, you can regain access after an account recovery waiting period, as outlined in this document: How to use account recovery when you can’t reset your Apple ID password - Apple Support

Instead of doing all that (which may be difficult at best), if you were give the original receipt, Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. Do this online at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not state what currently constitutes proof. Prior to this new online service Apple typically required the original receipt issued by an Apple Authorized dealer and documented provenance (so typically it would be the original owner who does this, or a descendant with legal proof of ownership).

Personally I would see if you can return the Watch since this is sounding like a lot of hassle to me.