From rummage sale, so called because it is a sale where you can freely "rummage" (or sort) through things in piles or boxes.
They typically include used clothes, old records, ancient appliances, Happy Meal toys, potholders, small brass deer, broken watches, coffee cups, and "crafty" Christmas decorations. Each sale is different, and finding something good is not a guarantee. (One might argue that it is actually unlikely, as you are sorting through someone else's cast-offs.)
It's main appeal is the thrill of the hunt and the chance at finding a real bargain.