system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do sellers on Amazon sell books for a penny?

So I've been buying a lot of books for college recently, and I noticed that  there is a lot of booksellers who sell books for 1 cent (plus $3.99 shipping). I'm really baffled by this, I can't see how they will turn a profit. Can anyone explain it?
gpt
Mostly to get rid of excess inventory and old editions.  You are basically paying $4 to throw the book away for them, and they are still making a little money on the S&H.  

Otherwise, they would have to ship the books back to the publisher (hardcover) or rip off the covers and ship those (paperback).  You are just saving them the hassle, and paying for the privilege.