This Saturday 9th, There is the annual 'book festival' in the Palermo area. I dont know much about it, (personally, reading is about as much fun as having cancer) but my girlfriend is going. I will try and get more details for you and im post them here when i do. Im sure you might be able to get rid of some there.
got rid of all mine at the book fair in caballito, also try round parque centenario. but they are quite picky with titles, and you get peanuts for them....so i would hold onto them or give them to a trusted friend. you could also go to hostels and exchange them, as another option.