: The seller claims that it was
: damaged during an un-authorized repair.

I would walk away from it. Why would you even consider buying a watch that the seller is already making excuses for problems with its condition?

Even if it is authentic, you should not settle for buying a damaged/misrepaired watch. You have no idea what other extent of problems it might have. Certainly you can do much better for your money.

Let this seller's problems with his watch become someone else's problems -- not yours.