I agree that you've got to distance yourself from this. See if you can get an email confirmation from the Best Buy rep you spoke with, stating that neither your nor your wife's iCloud account is linked to the watch, and there was no "original owner" before you, that it was a warranty replacement wiped clean and mailed straight from Apple. Since this rep says they information they can't share, I'm guessing it's information about earlier scams with this person.
Written confirmation can protect you if this escalates any further.
Written confirmation can protect you if this escalates any further.