Difficult situation . . .  did you have a specific return policy on the listing? 

I think at this point all you can do is to gather as much evidence on your end to support your claim, from photos and the listing to emails that have gone back and forth so you can back up your claims to ebay.   Put it in writing factually and in a timeline so you can make a succinct argument.  

Look at the buyer's feedback too, their recent bids, and perhaps even talk over your concerns with ebay live help before the stamps arrive back -which aside from giving you good info will also be on record  . . .  like this post I guess! 

And do give us an update 🙂