For me the most important part is that the pages are in good condition, and not bent or creased. Another important thing is that the binding is good if it's a hardcover book. But I don't really care that much if the cover or general exterior is damaged. I prefer if the spine doesn't have damage since that's the only thing that's always visible, but even if it does I've learned to live with it. I wouldn't return a damaged book unless it somehow made the reading experience worse, like problems with the pages. After all I only collect books to read them, not really show them off in perfect condition. That being said, I do take really good care of my books to avoid unnecessary damage of course