I can't wait to hear what happens. If I were you I would have listened to Bill (his first posting). If I were your buyer I'd write it off and give you neutral (what would be fair) or negative (if I were really upset) feedback. I know wht you are thinking: the open packaging might not be a problem, but I know waht the buyer is thinking: I'm not sure and I'll only find out when/ir my photos fail because of it.
To be fair, I did make it clear that the film was expired and listed it under 'used' condition. Anyway, I don't think he'll get back to me today. There was a couple of days between his first and second messages.