Don't buy on impulse. While there may be time limits for special offers, high pressure sales tactics can be a sign of fraud. Don't respond to the first offer you see. It's likely there will be many similar sites you can choose from. Check the actual price it will cost you to receive the goods.
Many offers are in foreign currency values and exchange rates can change quickly.
Also check the freight or delivery method costs and whether the goods attract sales tax or import duty. If you are buying goods which may need after-sales support find out if the seller has an agent in Australia and find out what their policy is on after sales service. Check the legality of owning or importing certain goods from overseas into Australia. Don't forget your local retailer. They may match prices on offer and may provide better service if goods need repair or replacement.