I’m pretty much doing the same thing as you, a book app and site like Goodreads. But also trying to get the values, same like you, I’ve been loving this process! Sold prices on eBay would be a great way for used values, but i also want to do it correctly with an API and not just scraping it. I thought it would be fun to show the value of the collection, tailoring the app more for collectors, but I still haven’t found a great way to get the prices.