Time Out says
On Sunday mornings, an avid mass of stamp and coin collectors swarm over the Plaza Mayor, buying, selling and eyeing up each others' wares. This also acts as a Sunday-morning attraction for anyone out on a stroll in town, whether or not they are really bothered about tarnished old pesetas, stamps of all nations and 19th-century share certificates. Other traders sell old magazines, second-hand books, postcards, badges and ex-Soviet bloc military regalia. You can even get phonecards nowadays, along with just about anything else that someone, somewhere, considers collectable.