If you can get a good second-hand copy, do it since this will save you hundreds of dollars. Just make sure that the edition number on the second hand textbook matches the one used in your lecture - you can get second hand bookshop from the second hand bookstore inside the Union Centre under the Tower building or by the flyers you'll see on campus. for law (and some accounting) subjects, i would buy new books since they seem to change every semester and sometimes corresponds by year.
I also say wait 1 or 2 weeks into the sem before buying the textbook, cos sometimes it ends up you don't use them at all or the textbook is just unreadable....and you end up wasting all that money.