i bought most of my books at start of uni last yr from book shwop. If you get in early, you'll most likely get your hands on books from popular subjects, such as econ, accg, comp etc. if you don't get in early, you'll be confronted with older editions, books with lower quality, or books with higher prices...
another way to buy your books is by looking at posters stuck on walls around campus. some ppl choose to sell their books this way because book shwop charges the seller a % of the book sold.
the best way from my experience is to buy off people you know. you might not know many people right now because you havent started uni, but it should be ok next semester. a good way to know people in grades above you is through your student society. or even ask your mentors, they can hook you up with ppl who wanna sell books.