It sounds like an interesting idea, but would be awfully hard to build up traction without some form of perks, plus policing it could be a problem against spam - don't underestimate how damaging bots can be.
Spam and bots aren't a worry for me technically (worked on plenty of systems before) but policing honest uploads is of course an issue. The ISBN can be checked and authenticated, so can the author name against the profile name to ensure those can only upload their own books. A report function would be handy to flag anything else misleading.
Perks wise, it would be a gateway. The author would be able to add details about giveaways, free stuff, and anything else that would grab attention. The website's function is to put talented, undiscovered books out there. I know of a few places to do this, and they have plenty of users...behind a paywall. I firmly believe it should be free to give everyone a fair chance.
Are there readers who favor self-published books over trad? Not people who read self-published books or like particular self-published authors but actively prefer self-published books as a concept over stuff produced by the Big Five. And passionate enough to go to a site other than Amazon or Goodreads to leave reviews. Because that's the constituency for this concept.
Well, that was exactly my initial question: is the market for this big enough? Firstly, the self-published market is huge. Secondly, platforms for discovering and supporting hidden talent like Patreon are huge. So in theory, people will want to use a platform to find small self-published authors, contact them, and support their work. That's why I think sites like Wattpad have thrived. But where there are sites like Wattpad for rough, unpublished stories, and bookstores/major websites for big house books, there's this grey area in-between.
The fact that authors can put up their own deals and even freebies is a huge draw, as I know there are thousands of people out there registered for various newsletters and paid platforms that share self-published work. The website could have a "free books this week" section etc to draw more users.
How would casual readers discover this site to read these reviews? Would they give them more weight than the reviews on Amazon, Goodreads, Bookbub, etc.? You could use the reviews from the self-published only site as editorial reviews on Amazon.
Review wise, the website could initially pull in Amazon reviews automatically which would save building a review system that people are unlikely to use early on, though I would need to add a simple function such as "likes" to add weight on the platform itself. It's disappointing that I'd have to lean on Amazon, but I'd just be making it impossible for myself without their data to begin with. Goodreads had a similar system you could pull data from, but as of December 2020 they shut it down. Openlibrary is great for data, but they lack substantial reviews even for popular books.
You would also have to vet every book put on the site. How would you handle hybrid authors? If an author has nine trad books and one self-published book what shows up on the site? Or books that were originally self-published but were picked up by a trad publisher? Or dropped by a trad publisher and became self-published. At what point does a self-published author producing work through a series of collaborations become a small press?
Hybrid authors is a good question. My initial idea is to take the ISBN data and vet it on a book basis rather than an author basis. The ISBN can be denied based on certain publishers. Small press collaborations and all the other possible in-betweens may be something to manually vet as the website grows before a good automatic process can be put in place.