> The Gloucester-based bookseller was founded in 2004 by Stuart Felton and Andrew Crawford, a former Amazon employee, with the mantra of “selling ‘less of more’ rather than ‘more of less’”. It aimed to sell 6m titles covering a wide variety of genres and topics, as opposed to focusing solely on bestsellers. [0]
Seeing as Book Depository later became a subsidiary of Amazon, it's fair to question whether that model held true. I don't have enough insight into Amazon's catalogue to say either way. Also, I can't remember to which degree Amazon relied on "other sellers" back in 2004 or if that came later ("other sellers" being a way to offer a wider selection than what one carries oneself in retail).
To answer your question for an example of a title Amazon doesn't have; I recently found myself recently looking for "Hur svårt kan det va'" by Eva Bodfäldt, which may admittedly be a contrived example, although Amazon does carry another title in Swedish by the same author [1] (though used and from a third party). As it happens, Book Depository has the title... alas, it's not available for delivery anymore.
[0] - https://www.theguardian.com/books/2023/apr/04/amazon-to-clos... [1] - https://a.co/d/bIEwL9W [2] - https://www.bookdepository.com/Hur-svårt-kan-det-va-en-liten...