While I'd never do it, there are examples of booksellers who do make a small profit from selling penny books on Amazon.
Who are they? How.... and why... do they do it?
Here's an article from several years back that delved into this mystery, and I think it's worthwhile to reflect on as pricing online has changed during the past year:
"Can you really make a living by selling used books on Amazon for a penny?" on the Guardian.com website.
Like I said earlier: I'd never do it. But some people are making money doing it. It's an interesting read. I especially like the story of how one seller got started with free book inventory.
I'd like to try that!