Are you disappointed if you buy a used book online and when it comes in the mail there's pen scratched on the cover? Or if a page is slightly torn?
I can understand being frustrated if a page is missing. You're not getting a full product. But if you can still read everything fine, is it really that bad? I know some people take special care not to scuff or ding any books and make sure to buy them with as little wear as possible. It makes them more appealing cosmetically, and depending on what the damage is it could effect their durability.
I've noticed I have this weird thing with used books where I treat them like an orphaned puppy. Almost like "Well, I'm taking it in, it's going to a good home. At least I'll take care of it and it's not collecting dust somewhere".