If the item is saying sold on your selling page, click the drop down at the right of the item, and click on view order details. That will give you the buyer's name and address. If its saying anywhere outside Australia, then you are likely being scammed.
Everyone starts with 0 feedback and most new buyers will buy something the day they create their account. Chances are they are legit, BUT, if you have red flags, and alarm bells going off, cancel the sale and use the option that there is something wrong with the buyer's address.
If they turn around and neg you, then you can ditch this account and create a new one. It's not like you'd be losing a stack of feedback.
As for the 21 day hold, as Padi said, you MUST post if the payment has been made. You are in violation if you don't. If you can't afford the $15 postage you had listed, then you'd be better off on Facebook, with cash on pick up. All new sellers are subject to the hold, as are long term sellers who have a year or so off.
Where is your buyer located? You can check on their feedback page where they registered the account, and also as mentioned above, in the view order details. As the item is showing as sold when I checked your page, you should be able to see where they are.
