The exchange bookshop in Dalkey will buy good quality secondhand books, particularly more recent releases. Also the secondhand bookshop next to Weirs pub in Dun Laoghaire and Eamonn's bookshop in Sandycove.
Wherever you take them, here's a tip: only bring in a few at a time. A friend of mine used to exchange books for credit in Vibes & Scribes in Cork, but found she got a better deal per book when bringing in only four books for exchange than say, a stack of twelve books.