Interesting point. If the deal had gone through, he had received the book, noticed the stain, ordered the grading notes to confirm that it was a stain, and then asked to return the book, then what? Seems like there is some ambiguity here particularly if the seller indicated in the thread that he didn't think there was a stain.
If you submitted the book raw, which I assume you did, why did you tell him something different when he asked about the stain? I mean if you sent the book in I'm sure you looked it over with a fine tooth comb right.