Oh dear.
The Resolution Centre is at the bottom of this page.
You have 30 days (no more) from delivery to open a Not As Described dispute.
That window has closed.
You have 180 days from Payment to open a Not As Described dispute on Paypal.
That window has also closed.
If you back your PP account with a credit card, you can try phoning the 1-800 number on the back of the card and enquiring about a chargeback. Card policies vary, but few allow chargebacks over six months.
Did you return the defective stamp to the seller?
If so, well..... you might try a Small Claims Court dispute in his location.
The last one I was involved in was in the 1980s and the filing cost was $25 but DD was involved in one in BC recently. She was the defendant and didn't pay anything, so I don't know how much filing was.
The good part was that the entire dispute was handled online, so location became moot.
If you did not, your best bet would be to resell the defective , but properly described stamp here and recoup at least some of your money.
Another thought, are you or the seller members of the American Philatelic Society? They have a dispute process and publish the names of uncooperative members in their magazine.