Is the seller responsible, yes. Within his return period offered, he has a 30 day return policy which is ample time to check the book, and return if needed. Being you never checked and you are no longer within the specified return window anything he does for you is him trying to do what he feels is right, but to expect him to jump through hoops for an ebay buyer all this time later isnt really reasonable either. My opinion is take the $100 and make sure you do your work in the future, and this isnt disrespectful as we have all made similar mistakes.