Are you aware that La Trobe Student Union has a second-hand book store? This is a not-for-profit service run by the La Trobe Student Union that enables students to buy cheap textbooks and earn extra money by selling old/unused books.
You can buy online or at the store on campus. The LTSU second-hand bookshop is located in the Upper Agora West (behind the Student Lounge) to the right of the Reception desk at Melbourne Campus. LTSU Members get 10% off!