International selling is generally safe with the type of item you're offering. (However, if you're selling a top-of-the-range laptop, and someone from Nigeria or Ghana suggests an unusual method of payment, it's almost certainly a scam).
If posting something to Australia, it's essential to attach a CN22 customs declaration. (Available, free of charge, at any post office). To qualify for the 'small packet' rate (which is cheaper than 'letter rate') it's also essential to mark the words 'Small Packet' at the top left hand corner of the addressed side of the package.
Surface mail can take up to 22 weeks for delivery, so it's probably not suitable for your purposes. Prices are here:
http://www.royalmail.com/portal/rm/content1?me diaId=400393&catId=400037
If you use airmail, you should either use specially printed packaging (available, for a fee, from any post office) or attach an 'Air Mail / Par Avion' sticker (free from any post office). Prices are here:
http://www.royalmail.com/portal/rm/content1?me diaId=53800712&catId=400036
If you want to get a signature for the package (i.e. the international equivalent of 'recorded delivery') it will cost you an extra �3.70 for the 'International Signed For' service:
http://www.royalmail.com/portal/rm/content1?me diaId=22700572&catId=400035
Chris