The shipping charge covers whatever meager profit they get.

Something like the profitability of collecting cans for the deposit.

Plus any return doesn't refund the shipping charge.

Quite a lot of them. The publishers are quite upset at the price collapse of used books as people go grab those up.

More like things have become so efficient, so that people donating their used books can have them go to a warehouse and get binned as having value and end up on the marketplace because there's $1+ worth of value after shipping.