I can't say that I've ever gotten a deal on a 35mm camera at a yard sale, but it's not for lack of trying. I was inspired to check out yard sales and flea markets by a story that I heard from a lab tech at college, claiming to have purchased his Olympus OM-3 for $8 at a yard sale (this would have been 1993 or so). The seller thought it was broken, but it simply didn't have a lens mounted, there was nothing actually wrong with it.
I was once offered a "Time Magazine 35mm camera" for $10 at a yard sale, but I declined.
That's sort of how I got my Mamiya 645 body and 80mm lens. I paid $20 for it "broken" and got lucky: it just needed a new battery.