I remember buying some cookies from such a fleemarket last year. Don't know what the cause was, but the cookies were good and the little girls selling said that they backed them.
In Germany there were also markets like these, but with no noble cause involved. It was just stuff that people didn't need anymore at house and sold for making an extra euro.
The initiative is great both for the school orchestra and for people who would collect antiques or to find bargains that have a pretty good value.