in Lahore every Sunday you can go to anarkali bazaar where is a huge market of 2nd hand books ,
at the end of urdu bazar opposite to Mori gate there are always some stalls of 2nd hand books but they will bring to their shops in narrow streets ,if you cant go to these places there are many book shops in Gulberg main market where they ssell 2nd hand books ,but personally i will recommend anarkali footpath market on every sunday ,please try them ,they start at 20rs