I have impeccable feedback after 14 years on eBay (mainly as a buyer)
As I have pointed out countless times, these days it is IMPOSSIBLE for a buyer only to get a Negative or even a neutral -- 105% IMPOSSIBLE. Therefore EVERY buyer has 100% feedback.
I am sure you are not in the following camp but sadly many buyers lie, abuse, threaten, cheat, claim things never arrive, and get a full refund - and do it all week long and still have 100% perfect feedback.
Why? As it is a 100% cost to sellers
, not to ebay.
Cheats and cons are out there in vast numbers these days, due to the stupid paypal refund policies, and sellers have zero protection against them, even if they DO check feedback.
Re the Cert, tell him to go pound sand if he did not ask your leave or agreeance to get one.
Give him a refund for the stamp and that is all he can ask. All you offered was: "you would refund the purchase price for any reason, including shipping."
Any smart seller on ebay (or anywhere else) offers such debateable items "as is" of course.