You can also try La Librería de Lavapies on Calle Argumosa, 39 (Metros Atocha or Lavapies). They have only a small used book section. However, they take everything (Spanish, English, others). They do not pay you, but rather you set up an account and get 30% of the sales price when the books are sold (as credit). They do seem to eventually sell all the books (at least the ones I've brought in) and I've been able to get a few second-hand books for free with the credit. I read (I think on their website) that J&J will only take books less than 5 years old (but I cannot confirm if this is true, since I haven't been there yet. It's just not near where I tend to go, but it sounds like a great place).