I don't think there's any set procedure for this situation.
The authenticators don't normally enter into discussion with the seller. I doubt whether you can do anything until the watch is returned to you, if rejected. Provided it's the same watch you delivered you will hopefully be back to square one again, with nothing lost.
If the buyer still agrees to accept it despite a minor defect the ball will be in their side of the court. But I don't see how they could win a claim against you that it's not the watch you delivered when it's been through eBay's own authentcation process.
As so often, the difficulty is in getting anyone at eBay to take ownership of problems. Hopefully the authenticator will sort out the problem for themselves
