There is a non-profit organization here in Spain called HUMANA
which operates several second hand clothing stores in Barcelona, Madrid and Granada. If you follow the above link, it will take you directly to a page that has the addresses to the stores in each of the cities.
Of course there is a bit of controversy surrounding this non-profit...there is an interesting forum of opinions HERE
... if your Spanish is up to par.
The premise of this non-profit (in case you are interested) is to collect donations of used clothing and then send it directly to Africa, retaining some of the clothing to sell here in Spain in their stores.
I think Spaniards, as a whole (and very generally speaking mind you) eschew the idea of buying/ wearing used clothing, so the market for such here is not as big as it is in the USA.