These questions are very similar since Waterstones are a company that sell books and your job will be selling books.
Try to showcase your experience in sales (if any), in a retail environment (if any) and your interest in literature. In addition, you'll want to show that you have good customer service and communication skills, together with the ability to work within a team.
With regards to why you want to join Waterstones; do some digging online. Look on Waterstone's website and find out some history about them or maybe some interesting facts, then link your answer to these - this will be a great way of showing them that you want to work for them, and not just any other job.
Good luck!