Second hand Apple Watch is not pairing to new device

This article explains the two ways the owner can remove from the account. If the Watch is still locked, the owner may have thought they removed it but has not successfully removed it from their iCloud account.

About Activation Lock on your Apple Watch - Apple Support

If it says it is locked to owner, it was NOT prepared properly and you simply cannot unlock the watch.

This is exactly what SHOULD have been done before you got the watch --> What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

Contained in the article are the following directions if they already gave the watch to you:

If you already gave away your Apple Watch, you can erase and turn off Activation Lock from iCloud.com. Here's how:

1. On a computer, go to Find My iPhone on iCloud.com and sign in with your Apple ID. 
2. Select All Devices, then click your Apple Watch.
3. Click Erase Apple Watch. Select Next until the device is erased.
4. Click the X button  next to your Apple Watch. 

If they are unable or unwilling to do this, you're the proud owner of a totally useless watch, which Apple will NOT unlock for you, nor can we help you unlock it.

If they can't do what is required, return it for refund.

Read this too --> https://discussions.apple.com/docs/DOC-250001038

What error message are you getting?

You may try this option. Set up your Apple Watch - Apple Support