depends on your uni.
the union at my uni provide a 2nd hand bookshop where students can sell their old books (for a certain % of the sale price), otherwise there should be noticeboards around campus with notices on.
there was another website *i think it was something like* www.textbookexchange.com.au (youll have to try a few variations out perhaps, that was just from memory) that provide the services you are looking for.