Is it possible to pair an apple watch if the person you bought it from doesn't know hos old icloud email and password?

Ask for a refund, because—without that Apple ID and password, or original purchase paperwork showing the serial number, or legal paperwork—it’s useless and worthless to you. Or recycle it, if the seller has disappeared.

Same issue as here: The All Too Common SAD Reality of Buying … - Apple Community

The person that you bought this watch from needs to follow all the steps in this link;

Remove the Activation Lock so that the buyer can use it.

If they can't or won't then get your money back or recycle the watch and put it down to a bad experience.