I bought a limited edition Kalevala for $4 and a few AD&D modules for $1 each at Goodwill. That’s about it for rare books, but it shows that the local Goodwill doesn’t pay much attention to book values. It’s just a matter of showing up at the right moment before someone else snatches the goods since it’s a busy store.
I’d like to point out that I spend a lot of time in used bookstores, and Goodwill-type stores, and I’ve bought hundreds if not thousands of used books…
… and not one has been a neglected Shakespeare first folio, or a five-buck copy of The Tales of Beedle the Bard. Nor are any of my finds autographed “Mark Twa-- ah,screw it, I’m Sam Clemens!”
Just books I wanted to read.
Really? Smug much?
Glad I didn’t tell the story about how I flipped a first edition, second state copy of As I Lay Dying, a first of Player Piano and another book plus cash in order to score a First edition do To Kill a Mockingbird.
<score!!!> 
No, actually the opposite. I keep looking for a “buried treasure” (local comic book guy’s term-- he found a rare Silver Age Superboy comic in a box he’d bought for $10). But I feel like a loser after all these stories.
Hmm, I would love to work my way up to smug. maybe I’m getting there, and I didn’t know.
But I love to live vicariously, so keep those first edition Harper Lee stories coming (now, if you’d only found a copy of the sequel, like I did…)
Sorry - please forgive my complete misread of your comment.
Fwiw, I had far more “finds” when I was shopping at small booksellers more than thrift stores. They’ve picked over a lot of stuff so the hit rate for interesting stuff is higher, but they still can undervalue things here and there. That’s how I got the As I Lay Dying for a fraction of its value and traded it towards TKaM for a lot more.
No harm, no foul… my last line could’ve seemed smug. But I do just buy stuff I want to read.
Drives my local comic book guy nuts. I’ll buy a Legion of Super-Heroes from the 60s and then roll it up and put it in my back pocket. I tell him that’s what I did when I was ten: “So I’m gonna jump on my Schwinn Sting-Ray and ride down to the malt shoppe, order a Grape Nehi, and actually read this!” Last time CBG said “So, you don’t want to buy a Mylar acid-free sleeve to go with that?”