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.
I've thought about this. I believe the cost of shipping will be paid for by the person shipping it out.
I am not an economist, but my idea is that even if someone is doing this, the economy works itself out because they still gave away a book that someone wanted.
And if the books are that garbage, nobody will request them.
Of course, this is all theory and I'll have to see/adjust as it happens.
But the swap is for free (+cost of shipping). So of no value to one of those kinds of entrepreneurs (unless maybe they find a way to arbitrage shipping costs).
The project looks cool and I wish you the best of luck in designing solutions to avoid bad actors.