I've been a bookseller, part-time, full-time, in huge shops and small ones.
I would say that most booksellers are over-qualified and extremely knowledgeable. But it is at the end of the day a shop job. Tilly works weekends. She is probably not having meetings with publishing reps (I don't think anyone from the shop floor probably is at Waterstones). It is a physical job (being on feet all day, carrying books) that requires dexterity of mind as well (do you know a blue book about Italy?), but ultimately is no better or worse than any other shop job (if you love the product).
She may help sell thousands of books but some of those are through affiliate links that are nothing to do with Waterstones.
I like the meme that says 'I wanted to end her career, not mine!'