I say he has to pay you for the difference of the stamp . So he owes you abut 24 cents . What is a 1st class stamp now anyways ?
I have a feeling that's part of the trolling strategy. Seller probably made the other 23 purchases themselves to make the item appear sought after.
He sold 20 to one person via best offer. It may be a legit transaction but obviously at nowhere near the asking price.