Apple watch

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

See this support document, the section on how to erase if you don't have your Apple Watch. Unpair and erase your Apple Watch - Apple Support

There's a section in the link I posted that tells you what to do if you no longer have the phone that the watch was paired too.