Before sell or trade in your apple watch you must prepare apple watch for sell, means remove all the data like Scanned cards, Photos, Wallpapers. And also your most important and traceable apple account, passcode set on your apple watch. Also not without remove your apple ID or passcode from device no one can setup own apple ID on old Apple watch or Pair with another iPhone.
Anyone planning for sell cheat apple watch to other, then that’s not possible or there is no any trick like you can do on iOS device (JB). 
Steps for Prepare apple watch for sell to Friends or other: Clear data and Setting
We know that apple watch without iPhone not start up, at the same time reverse condition also applied with out unpair we can’t leave apple watch sell for trading.
Same process you have to follow on apple watch send for service or repair at apple center or store.
Unpair apple watch automatically turn off activation lock, lets check how to do step by step
From iPhone
1. Keep your apple watch and iPhone close together, Launch watch app on iPhone.
2. Tap on My Watch tab from bottom screen.
3. on screen, you can see unpair apple watch seem like in below image. 
4. Enter apple ID passcode for successfully unpair. Forgot your apple ID? Reset apple watch.
That’s it.
From iCloud without iPhone prepare apple watch for sell
You need to remove apple watch from iCloud > My Devices section.
Go to iCloud.com > Sign in with Apple ID and Password > Settings > My Devices.
Tap on cross red mark and remove apple watch from list of apple device.
Many of my friends sell apple watch with setup on another iPhone commitment, Yes that time you need to pair apple watch with iPhone again.
Online Trade-In: #1 store for Sell Old Device on Gazelle
Want helps on unpair or Prepare apple watch for sell on any circumstances share with me on discussion section.