If you get a book back, it’s not a scam because it’s not designed for you to profit, either monetarily or materially. It’s obviously misleading saying you’ll get “up to” 36 books back, but that’s not guaranteed and shouldn’t be expected. If someone joined such an exchange (and it was trustworthy), they should think of it as a random book swap and expect to get a surprise book back in exchange for theirs. Anything extra is simply a bonus.
1 for 1: no one gets rich, and no one gets scammed, in theory…