apple watch activation lock removal

You will need to recover your original Apple ID so you can log into that account and remove the Activation Lock on your Watch. See:

If you forgot your Apple Account password - Apple Support

Apple will not help you remove Activation Lock. If you can't recover the Apple ID used to lock the Watch, then it is now useless.

Good luck.

There's two ways to remove a lock. One is to do as others have been suggesting. The second is not available to you based on the fact that you purchased this from a secondhand retailer. That would be to present Apple with the original Apple sales receipt and ask them to unlock it.

Recycle the Watch if you cannot recover your account information. From the very bottom of Apple Trade In - Apple :

"Does Apple offer recycling?

Yes. Apple Trade In lets you recycle any Apple device (including devices from Apple-owned brands) at any Apple Store and on Apple for free. That includes your batteries and old electronic products as well as free, on-demand packaging recycling for our commercial, education, and institutional customers. When we receive your device, it will be thoroughly inspected to determine if components can be recycled or reused. Whether recycled or reused, all activities relating to the processing of your device will be managed in an environmentally responsible way."

You certainly seemed to know what your Apple ID was when you setup the watch. Any particular reason why you suddenly can't remember it?