On ebay both cams ITR go for about $300-350. Now I know you did'nt know it was a b16 cam. But you did state that they where both ITR cams. It's your mistake for selling them as ITR cams. I would just tell him to send back the cams and you will refund money. Now the shipping part is up to you.
But on a second note the one is worth $300 on the books so he did get his moneys worth. But then again there was one thing stated and he got another.
Ehh it's up to you. I like to make my custumers happy so If I mess up I grant the custumer there money back.