The 2nd hand bookshops usually do not like to take in old books, unless they are in fantastic condition - and then they'll only offer a very low cost.
I have tried the ones in Far East Plaza (incl Sunny), Selegie, Bras Basah, Clementi... no one is helpful.
In the end, I packed all my library of story books (nearly 500 of them) into 2 large suitcases and dropped off at the Salvation Army at Bt Timah. Hopefully they put to good use...