You will have to make sure, that the Activation Lock has been removed by the previous owner.
Check this article about it:
About Activation Lock on your Apple Watch - Apple Support
If the Activation Lock has been removed, the Apple Watch can be paired with your iPhone and Apple Account.
Also, depending on the age and use of the watch, the battery health might be near or under 80% and you will have to get the battery replaced by Apple in the near future.
And, depending on the age, the watch might not be water resistant anymore:
About Apple Watch water resistance - Apple Support
You can find an estimate for getting the battery replaced on this Apple Watch Service page:
Apple Watch Service and Repair - Apple Support