I can’t contact the seller to unlock my watch

It sounds like you are talking about iCloud Activation Lock. Only the owner of the Apple ID can remove it. You’ll need to keep trying to get in contact with them.

https://support.apple.com/en-us/HT205009

The person that you bought this watch from needs to follow all the steps in this link;

Remove the Activation Lock so that the buyer can use it.

If they can't or won't then get your money back or recycle the watch and put it down to experience.