Bookii : Sell & Purchase Bookslazydevelopers
1.0 Varies with device
Gather your old books and start uploading details on this app. Soon a buyer will contact you and buy your old stuff.
No money to buy a new book. No problem. Just request for a book you want and you will be notified as soon as similar book as your request is uploaded.
Just bring your old and unuseful stuff here and make it useful for someone else.