You'll notice that it's the experienced sellers who mostly do not leave feedback straight away. This is because any nong can pay for something promptly... and feedback for this is only a small part of rating the buyer.
Much more important is how the buyer communicates... especially in the event of Australia Post problems where the item is held up.. or even worse, damaged! It is in this situation that both buyer and seller should be rated. Problem resolution qualities are by far the most important aspect of ebay participants... and therefore I only ever leave feedback once the deal is completely finished and both parties are happy... or at least agree to disagree!

Cheers,
Chris