David Isaak, author of Shock and Awe, has this to say about buying used paperbacks instead of new books for which an author receives a royalty:
I find this too be a gray area, not only ethically, but in terms of my own preferences. To put it simply, more money is good…but more readers is even better, and I’m not sure where the trade-off lies.
In the long run, I think any author wants his or her books to be read. This will be particularly true of authors who sell few books. If you’re selling by the cart load, you’ve probably making enough money not to worry about that secondhand 10% or so.