* You already own it: Maybe you have a physical copy of the book or have already purchased it digitally.
* You've already read it: You might have already read the book and are not interested in reading it again.
* You prefer other books: Maybe you have found other books that you enjoy more than Harry Potter.
* You are waiting for the next book: If you are a big fan of the series, you may be waiting for the release of a new book and not interested in reading the older ones.
* You are trying to avoid spoilers: If you are planning to watch the movie adaptation of the book, you may want to avoid reading it first so you don't spoil the story.
If you are looking for other reasons to avoid renting the book online, you can provide more context and I can give you a more specific answer.