A seller can add whatever terms and condition he or she likes to an item for sale, however, if they break Australian consumer laws, they aren't worth the paper they are written on.
Bag inspections as a condition of entry into stores is a good example, there is nothing in consumer law that requires you to present your bag for inspection upon leaving a store, but nearly every grocery store has a sign at the entrance and checkout stating this condition.
The worst that they can do is refuse you entry if you wont offer your bag for inspection, which unless you have a track record, they don't or wont do, because they know you would have legal recourse if their action is not warranted.