I don't know if you know the episode of Hancock's Half Hour-probably not-when Tony has got to the last page of a detective mystery only to find the last page has been torn out.
When checking books to sell we note the browning, pen marks, pages being ripped etc. but what happens if the book is made unreadable by missing a crucial page?
Obviously it is easy enough to check if the front pages are all there and the back pages. The next obvious place for pages to go missing-and sometimes entire chapters do-is the middle.
Of course if you are selling a book in mint condition and it turns out to have pages missing it is no longer considered so. But then again, it can no longer be called 'excellent' or even 'good' either.
Of course the only real way to check there are no missing pages is to read the whole thing, but that may be impractable so the only real alternative is to offer a refund.
Unless it is a particularly valued customer it may not be a good idea to refund a buyer of a book in poor condition who has found there are pages missing.
Pages are not the only thing to go missing of course, illustrations can go missing as well, sometimes torn out, sometimes cut out. Again it can be hard to check, so offer a refund.
Let's hope all your reading is uninterrupted.