Okay.

If that's the case then when you (@dabea-7654) get the watch back you should check the serial number for yourself. (Take it to a trusted jewellers if you don't feel confident in removing the backing yourself.)

If it does match then I would report the buyer for abuse of the returns process.

If it does not match then I would ask eBay how either your buyer managed to return a different watch through the authenticators, or how the authenticators failed on their authentication of the watch originally. Because it can only be one of those two scenarios. Neither of which are good.