I'v read that the Madrid has the biggest flea market in the world. Is it on Sunday? Is it absolute mayhem, and something to avoid, or is it worth it for me and flea market minded mother to check it out? Vale la pena, o no?

Also, does anyone know if there is anything like second hand shops, or charity shops, or consignment shops where one might purchase good quality clothing at a lower price? Specifically in Barcelona and Seville, but generally does the concept exist in Spain?