I've bought most of Barbara's books from the local second-hand shop, but that's because most of them I could only get second-hand, or would have had to pay a huge premium postage-wise to be able to buy new. I have a lot of other books that I bought second-hand because they were either out of print or horribly difficult to get hold of new. A few were even available new, but I wanted an older printing to match the covers on the rest of the set I already had.
Back when I was very young and didn't have much money, I couldn't afford to pay new prices for many books. I wouldn't risk that sort of money on an unfamiliar author. That's when I was *really* grateful for libraries and second-hand shops. It allowed me to explore and find new authors I liked, without having to risk a lot of money. I still do that. So as far as my purchasing's concerned, there are authors who have made sales of new books because I was able to get something of theirs from the library or second-hand.