In recent years, I've found that Amazon's Kindle store has become less good. Their recommendations are much worse than they used to be, their handling of book series is bad (if a book is in a series, show me them all, in order, and identify those I already own), the pages are just awfully messy and cluttered, they push Prime in my face all the time, etc, etc.
I'll love a great, curated online book store. Much like a real bookstore, have things organized well, have staff recommendations (not random bot-boosted rubbish); focus on authors with interviews, and AMAs, and so on; and make it a nice place to browse (and buy).
If you make the commercial deal for authors/publishers decent, I suspect you would be able to build a decent list of suppliers. And if you focus on the "good bookstore" feel, I think you've got a good chance at some success.