Interesting discussion.
Given how it has been presented - the dealer took a swing and missed badly. That is not the sellers issue. The dealer is the PRO making a living off buying and selling coins. If nothing has been left out of the "story" then his (dealer) loss and no obligation from seller to give anything back.
As my add on - had the dealer waited 6 months to send the coin in and came back with same results - he still sending the same letter to the guy? That is silly. BUYER BEWARE goes both ways.