As you'll be at UOW, also be sure to check out the UniClassifieds
book section, especially closer to the start of session. Old 1st year students want to sell their books just as much as you want to buy them cheap. Give the noticeboards on campus a glance during enrolment/O-week, as you'd probably see a few advertised there as well.
But if you're set on getting them in advance/with less risk, I'd say go with Frigid's advice above. I've heard positive reports from users of both sites.