Yes you came out ahead this time.
That said, did you refer the buyer to the PF's Expertizing Term and Condition Standard for Stamps? I refer to the one as quoted below: Never Hinged: Full original gum as applied by the printer and which is free from any disturbance.
Cognizance will be taken of the normal gum condition for each issue. Gum skips, bubbles or other natural gum irregularities including short gumming will be cited where these are considered to be significant.
This information and much more can be found at the PF's list of expertizing term definitions: https://www.philatelicfoundation.or...izing-terms/
Perhaps if you point your buyer to this information, he may better understand the language of the PF Certs he receives and thus allowing education from other than the dealer from which he has taken "counsel."