There are plenty reasons to visit Marché aux puces de Saint-Ouen if you're ever in Paris: mid-century travel brochures, a Futuro house, mid-century furniture, more mid-century furniture and mid-century modern furniture. If you do go, plan of spending a least a few hours there. We spent about four hours looking and shopping and didn't even scratch the surface. My guess is if you wanted to do the whole thing, it would take days.
Here are the last of my photo collection from Marché aux puces de Saint-Ouen: