How to remove activation lock from Apple Watch?

That's an Activation Lock;

About Activation Lock on your Apple Watch - Apple Support

And to remove the lock the person you bought the watch off has to follow these steps;

What to do before you sell, give away or trade in your Apple Watch, or buy one from someone else – Apple Support

What kind of lock,passcode lock,activation lock or password lock?.

I would get your money back because if that Activation lock isn't removed you will never be able to use the watch.

If the shop won't help it might be because they are stolen watches.