I know this was your first sale onebay and hopefully it will end up to be profitable. In your listing if you stated true facts, no returns unless DOA and it was not DOA I would not refund the money. If you wait to see if a case is opened both ebay and PP will contact you. You can explain this all to them and if they find for the customer you still can do the refund if you like.
I would write an email to eBay and explain the whole thing to them. They will be able to steer you in the best direction. Emailing the buyer might not be the best way to go about this. I doubt his reply will be what you expect and that will only make matters worse.