Here’s the summary (and this is my take alone):
1. If you can afford to buy new books, that’s an excellent way to support the work of authors you like best.
2. If you can’t afford to buy new books, the next best way is to borrow from the library. Libraries deserve support. Libraries also support authors.
3. There are times and situations in which buying a used book is reasonable. If the work is out of copyright or out of print and/or if the author has been dead for a while.
If I buy a copy of a book new, I am then comfortable buying a second copy used if it’s for my own use.
If the author is new to me, I will get his or her work out of the library until I decide whether or not I want to purchase the books new.
Once I decide to buy a used book, I will try to get it from a nonprofit. For example, a library or school sale. There are a number of organizations that collect and sell used books for non-profits. The most visible one is Better World Books.
These are my guidelines. Everybody has to decide for themselves how best to proceed.