The answers to your question depend a lot on the individual supplier, the product, demand for the product and your estimated profit margin.
Firstly you need to research your market to ensure that the product is in demand, not available elsewhere at a price you cannot match (or get close to matching). You also need to take into consideration customs duties when importing.
Yes, selling a product from outside the country that is delivered directly to your customer is importing and, depending on the value of the item, could be liable for 10% GST. You customer would be asked to pay this before they could receive the item if import duty was required.
Time of delivery depends entirely on the means of delivery, the time it takes for the supplier to despatch.
If you have found product that is in high demand and not readily available in Australia or your country of selling you may have found a profitable niche and it would be worth considering. Perhaps buying a couple of the item and listing them to check demand would be a first step.
If you are selling on eBay (or even with your own site) it might be worthwhile checking out Auction Inspector
. This would allow you to carry out research to find out if the item you are considering is worthwhile. You may even find a niche market for another product.