Factory resetting used Apple Watch

I bought an Apple Watch series 3 on eBay, it was advertised as a locked device and I made sure to look into resetting methods before purchasing. However when I received it today and tried to do the factory reset it isn’t working. I get to the power off screen, i hard press the power off bar and press the following confirmation button after connecting it to a power cord. It goes to the loading screen as if it’s working but then after a few minutes it goes back to the lock screen with the old users photo and it’s as if I did nothing at all. What are my options?