You can try the steps in the "How to erase your Apple Watch if you don't have your iPhone" section of:
Unpair and erase your Apple Watch - Apple Support
But the watch is probably Activation Locked. Activation lock is an anti-theft measure that cannot be bypassed. If the original owner cannot or will not remove the lock then the watch is useless to you or anyone else. Try to get a refund. For more info, see the "Before you buy a watch from a third-party" section in:
What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support