One of the cool things about collecting pretty much anything is the rare occasion where you unearth something cool at a flea market. Don MacPherson recently did a review about an issue of Green Lantern he found at a flea market.
I don't think I've ever actually found anything really cool, comic book-wise, at a flea market. The best I have ever done was at an antique store that had a pile of comic books really cheap (some good stuff, too).
How about you folks? Share with me the coolest comic book purchase you've made at a flea market, yard sale, etc!