OK so all I know about comic grading I learned in a few minutes on Google.....
I take it a 5.0 grading is not very good (at least has some major defects)? Not sure why you didn't include more specific details to describe the major defect (married pages) in the listing rather than after the fact.
but he was informed of this like everyone else, just after he won the auction
This sounds like you told the buyer "thanks for buying oh by the way here's the defect I didn't mention in my listing".
This is an experienced buyer, perhaps they should have known what a 5.0 grading means but it's somewhat irrelevant. Since they paid via PayPal they will have no problem sending it back to you and getting a full refund. As Monique implies, if you don't stay on good terms with the buyer you could have a serious loss. On the positive side if they get a refund through PayPal you will get your fees back and if you move quickly you can still get your eBay FVF's refunded (time is running out quickly for that).

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915
- answering Trolls is voluntary, my policy is not to participate.

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915
- answering Trolls is voluntary, my policy is not to participate.