I didn't know "El Rastro" was the biggest flea market in the world.
It takes place on Sundays mornings and bank holidays mornings. But this only refers to booths. The regular shops open also during weekdays.
You can download a good Madrid map from my site at http://www.chelo.com
. Look for "La Latina" subway station at the left bottom corner. "Plaza de Cascorro" and "Ribera de Curtidores" are the main Rastro streets.
As for second hand shops, you can find some at El Rastro. However, as far as I know they are not popular in Spain. In fact, there is one at 15, Hortaleza and their products are even more expensive than new ones!!.
The best tips from your favourite hostal in Madrid.
Hostal Chelo at http://www.chelo.com