For a while, I would get my dad machinist manuals from this awesome old bookshop in Bellingham. They had things like "motorist's repair manuals" from 1902 and at one point I think I got him a small engine repair manual from 1905. It talked about the "interesting developments in internal combustion from Dr. Diesel in Germany" and shit. He put them on a shelf and never looked at them. I may have to reclaim them at some point.
My best used bookstore find was a copy of John Playfair's Elements of Geometry, of Playfair's Axiom fame, sitting on a metal shelf outside fading in the sun with all the other books they didn't think they could sell. I assume they thought "just an old textbook" and didn't bother to look it up. I gave it to the university library, because I was no collector and didn't have a clue what to do with a book like that. They gave me a receipt for around $3,000. I kind of wish I'd held on to it now, because there isn't much math history you can hold in your hands, but the tax refund did pay most of my tuition for a semester. If I ever decide to start collecting something, old Euclids will probably be it.
Mine doesn't really count, because it was Amazon. For reasons related to writing, I needed a copy of this book, which is esoteric to say the least, but hey - if you need to know about all the designs that didn't get made into the SR-71, it's the book to have (especially as two are still classified 60 years later). It's basically a paean to Kelly Johnson and his merry band of Skunkworkers with a lot of cool drawings in it that you don't see anywhere else. It is not a cheap book, nor one that exists in many copies, but I lucked out - across town, in Burbank, Amazon had a "used, some markings" edition for $7. I had it shipped because Prime even though I coulda picked it up with a one-hour drive. It showed up two days later. With the author's signature. In a dedication. To Kelly Johnson's widow. Thanks, Amazon, for the generous offer of $8.42 to buy it back... but cold dead hands.