Yes the places that buy used books respect the stamp. They usually take all the books and take them to some place Half Priced Books https://www.hpb.com/ for some quick cash.
Places like that won't buy them if they are stamped.
They could try selling them on Amazon or E-Bay, but that's too much effort for quick cash to get a fix.
I build these and have one at my house.
Its been interesting.
Had some teenagers try to blow it up with fireworks.
Have to constantly remove proselytizing, mostly christian, pamphlets from it.
Had to buy a stamp https://littlefreelibrary.myshopify.com/products/self-inking... so drug addicts don't clear it out sell the books to buy smack.
Other than hat its been mostly self sustaining.