I bought Apple Watch from this person , how do I get their Apple ID unlinked without having their information?

Try to get the owner to remove it from the Find My application on their  Devices.

if not then the watch is basically useless

Hello:

Sounds like you’ve encountered the Activation Lock. It’s a safeguard which makes the device wortless if it’s stolen.

The bottom line is that the seller has too cooperate.

Otherwise, you have a paperweight.

This article provides all the detail:

https://support.apple.com/en-us/HT201441

Here’s a little more info, specifically about the watch:

https://support.apple.com/en-au/HT205009

You can't. The watch is worthless unless you can get them to sign out/unpair the watch from their Apple ID.