Tom:
From your description of events, you sent him pics and told him what the box fit.
He decided to jump on the purchase.
His decision, not yours.
You went further on the deal, he can either sit on it, resell it or punt it around his yard.
It's not your problem anymore.
I needed the Driver-side door mirror for my '76 450SLC, and got a LARGER mirror...hell, it may fit my 280E, but it didn't fit the 450SLC. BFD for me. I'm not going to hassle the guy for my $15.00 bucks back...geesh...
And if someone else needs the mirror, I'd sell it to them for the same $15 that I paid (plus the shipping...one way) and consider the deal done.
Life's too short to continue to haggle over used car parts...
And again...he had your pics to make a decision...let him live with it...you did OK in our book...