From other posts in the forum, including one in the last few days, it is clear that Ebay accept return cases if buyers claim the item is not as described.  (Authenticated items)

The recent one I saw was for trainers which were 'too narrow' for the buyer.

The item went back to the authenticator and was returned to the seller for a refund.

What has happened here does not mean the watch is not genuine.  In your listing, you state, 'To ensure this piece is in top-tier condition, I had it sent to the IWC Flagship Boutique on Bond Street, London in March 2025 for a full factory sapphire crystal replacement.'  This would not have been carried out if the watch was a fake.

You mention scratches - if there were scratches on the back of the case, not mentioned or photographed, that would certainly be the basis for a return.

The watch is on its way back to you.  Once it arrives,   you will have 48 hours to refund the buyer before they can ask Ebay to step in.

You are too late to open an Ebay case against the person who sold the watch to you.