Legally, yes you can. See customs.gov.au - they talk about Australian companies who post directly from wareshouses overseas. Perfectly legal.
The same rules as sending anything overeas, or receiving in Australia, apply.
e.g. you will need to write on the customs form that it is "*merchandise". If you tick "gift" or anything else, its illegal, and you and your company can be in big trouble.
e.g. When your parcel arrives, wherever, the person receiving it MIGHT have to pay import taxes. etc etc
If you are posting to a customer from another country, you just need to put where the item is located in the listing (hence where it will be posted from).
Many sellers on Ebay do this.
What you CANT do on Ebay (as far as I know, someone might correct me), is put a listing up for an item you dont actually have, sell the item, then go to a company overseas and buy the item, and send it directly from that site to the buyers address.
