What if the seller won’t unlock it for you?

Sorry, but only the previous owner can unlock the Apple Watch.

About Activation Lock on your Apple Watch - Apple Support

Apple Support might be able to help you, if you can show them the original purchase receipt of the Apple Watch.

Apple will only unlock devices for the original purchaser, so they won't help you. Return it for a refund if possible.

Then the watch is of no use to you.