Hi
You appear to be referring to Activation Lock, which prevents unauthorised use of Apple Watch if it is lost or stolen.
If the Apple Watch is locked by Activation Lock, then it is not necessarily stolen.
The previous owner may instead not have realised that Activation Lock remained enabled or understood how to turn it off before selling the device.
To be able to pair the Apple Watch to your own iPhone, you would need the previous owner (who may not be the seller) to either enter their Apple ID and password on your iPhone or to turn the feature off via https://www.icloud.com/ (by signing in with their Apple ID > Find My iPhone > All Devices, then select the Apple Watch > Erase Apple Watch > Next until erased > click X next to the watch).
You will not otherwise be able to use the Apple Watch. There is no way around this security feature.
Apple will not advise you of the previous owner's contact details, pass your details on to them, unlock the Apple Watch or enable you to trade it in.
If you believe that the Apple Watch is, in fact, stolen, then I suggest that you hand it in to the police.
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