I always laugh when I see the excuse that someone bid on something by accident. Thats like driving off in someone elses car by accident. Kinda funny.
It looks like he bid on it by accident twice, according to the bid history. I would say, if you get the card back in the original condition, give the refund. Also, I would then contact the next highest bidder if their bid was close to the winning bid or just relist it. In my opinion, the price is fair for the grade.