This feels like one of those things that will be ruined by bad actors. Finding cheap worthless books for free or next to free is quite easy. I could get my hands on 10K "books" right now for the cost of picking them up and storing them. If there is anything valuable on your site it will likely be quickly swapped out for garbage by some "entrepreneur" flipping them to a for sale site.

The project looks cool and I wish you the best of luck in designing solutions to avoid bad actors.