If you want to sell a second-hand text book, you can place posters on the designated student notice boards advertising your item. You will need University of South Australia Student Association to stamp the poster with approval otherwise they will be removed by security. Likewise, to buy a second-hand text book, you can check the noticeboards around campus.
The University of South Australia Student Association also operates an online second-hand textbook store. You can drop off the books you'd like to sell at any of the University of South Australia Student Association campus offices.
For students unable to attend a metro campus, Textbookexchange offers a free (signup required) national service for buying and selling second hand textbooks. Note that this site is not affiliated with UniSA or University of South Australia Student Association.