Apple Watch
I bought an I watch at a flea market the guy said it was unlocked but it isant.. I don’t know the Apple Watch
Apple Watch Series 3
Question marked as
Try pairing it to your iPhone (you must have an iPhone)
Set up your Apple Watch – Apple Support
If you are being asked for the password to an apple ID email address that you don't recognise then it is activation locked. It is totally useless to anyone unless the previous owner removes it from their account. Same applies to almost all apple products apart from the ipod classic.
Dispose of it responsibly.