I have long thought about, alternately looking forward to, and dreading this day.
I have, at long last, found one of my books in a used book stall.
On the one hand, it means that someone once bought the book, and that the stall owner feels that someone will still pay money for the privilege of owning and reading it. On the other hand, the original owner didn’t prize it highly enough to keep it.
On the plus side, it’s definitely high visibility venue. The book was prominently displayed (or at least wasn’t hidden away, only the spine shoeing). And two books by a fellow writer (we’re both in two of the same writing groups) were nearby, so I was in good company.
It was Lost Wonderland, the one about the turn-of-the-century largest amusement park in New England.
With the stall owner’s permission, I autographed it. He was asking a fair price for the book.
Or maybe the original owner went to his or her reward and everything was cleared out of the estate? Ya gotta look for that silver lining! I was going to say that you need to look for the pony, but for the benefit of those who never heard that joke/story, I went for the obvious saying. 
Pretty cool, tho. Eons ago, I saw a kid wearing a Halloween costume that included a cape I’d made and eventually donated to a thrift store. That or someone somewhere had used the same pattern and fabric and color choices that I had used a decade or so before.
I give all of my books away after reading them once. I don’t save any of them no matter how much I liked them.
I’ve always had the attitude of “yay! the book will find another reader instead of sitting on a shelf somewhere”. Kinda weird since I had almost 10,000 books sitting on shelves myself, but I don’t pretend to be consistent.