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Put your account on vacation hold and wait until they either accept or reject it. If they accept it, you'll need to cancel the order and if they reject it, the book should go back to your bookshelf. Then you can delete it before going off vacation hold:
If you want to delete your entire account, you need to ask them by using this link:
The "Contact Us" link at the bottom of the page would be able to help you close your account.
If you posted a book that was wishlisted it is likely on wishlist hold waiting for the other member to accept. Once it is accepted you can then cancel the transaction.
To prevent it from rolling over to another member in case the first member doesn't accept you can put your account on vacation hold.