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Aaron M.I might be in the minority here, but when I offer an item everything I sell comes with a lifetime guarantee of authenticity (or in your case, the card not having been doctored).
That protects me from "buyer's remorse" or a disagreement over condition, but leaves me responsible if an item I sell turns out to be fake. I do this especially with vintage programs and pinbacks for which reproductions abound, because I want the buyer to be confident I am selling them authentic material.
I do this also, because that's the way I would want to be treated.
I bought a Christy Mathewson Memorial tag from Lelands earlier this year in one of their "Fredo" auctions. These were originally issued in the 1920's, but reproductions are rampant (Scott Gaynor had been, unknowingly I believe, selling a cache of these this spring.)
When I re-sold the item (months later) on E-Bay, the buyer upon receipt informed me that it was a reproduction. I had guaranteed authenticity in my listing, so I issued him a refund. (It was my responsibility, not Lelands, to refund the customer, since I sold it to him.)
With the information the buyer provided, plus some detailed background information Rob Lifson provided me, I went back to Lelands (again, months later) and gave them the information. They were extremely responsive and issued me a refund within two weeks.
To me, that was a text-book class way to respond and handle the situation.
So, my take would be it's Sandiegowill's responsibility to issue a refund to the buyer (because he sold him a trimmed card) and then Sandiegowill should pursue collection of its funds from the consignor. (Or list the card again as trimmed and retain the proceeds.)