How do I unlock an Apple Watch with a disabled Apple Account?

Get them to give you the original receipt of purchase and a bill of sale transferring ownership of the watch to you.

Make a Genius Bar appointment and take them along with the watch to the appointment to request Apple unlock the watch.

You can't do anything. Activation Lock prevents a lost or stolen Watch from being used by others. Only the original owner can remove the Lock through their Apple Account.

The fact that the seller sold you a watch with no access to the Apple Account to remove the Lock hints at the fact that you were scammed.

You will never be able to use this watch without access to the original owner's Apple Account.

M1 Mac mini, macOS 15.3.1;  Watch, series 10 GPS-only, watchOS 11.3.1; iPhone 16, iOS 18.3.1; TV 4th Gen, tvOS 18.3; HomePods, audioOS 18.3 and Airport Express w/ Airplay 2