Here's my latest score from paperbackswap.com. I'm amazed every time at what great condition these books are in. Although this one's been around since 1994, I'd say it's never been read.
Sometimes I think about where these books have been before landing in my library. Who bought them, and why? What shelf did they sit on, waiting to be read? How many owners have they had? Why are they being returned to circulation now?
Many very expensive books (signed copies, very small first printings, and so on) have a provenance, just like antiques. For the rest of them, we'll never know.