There are many things you need to check when you are buying.  The most important are listed as follows:

If you ring Australia Post or Customs and are told it will only cost you a certain amount, you must realize this is only for using the regular Australian Government post.  If you use a private courier company you may be charged much higher costs for customs and such.

Check with the seller as to what courier they will be using.   Depending on the courier it can have drastic consequences on your purchase.  For instance, when you import into Australia, you will ALWAYS go through customs no matter what you purchase or how you bring them into the country.  Whether or not you are charged for this depends on many factors.  Mainly, the actual value that the seller writes on the declared commercial invoice, and the courier costs to send the item to you are the governing factors.  These are the two fundamental amounts that are worked on.  For instance, straight off, your item may incur a 5% GST by the seller, from the country you are importing from.  This will be added to your total, plus the postage and handling cost that the seller charges you.  Secondly, you are then liable to be charged for a tarrif which could be anwhere from 5% to 30% depending on the items you are importing.  You will then also have to pay GST on the courier costs and GST on the value that the shipper has declared the items to be worth.  You will also have to pay for the brokerage fee of roughly $55, depending on who you nominate this can vary.  Then, after all that, you may have to pay for Quarantine, for example, if you buy golf clubs (they may have traces of dirt on them) 2nd hand shoes (likewise), seeds, food traces, plant materials, bones, leather; all these items can incur quarantine inspections and charges, anywhere from $50 up to $100's of dollars depending on whether they use fumigation, spray, etc.

So as you can see, just so far: Import Duty tarrifs, taxes, courier costs, GST, quarantine, customs handling, can all amount to alot of additional costs.

Having said all that however, there are huge benefits though, in that if you import an item or items under the limits of what the cut-off for inspection charges are, you may not have to pay any additional fees at all.  For instance, private courier companies usually set their threshold at around $250, whereas the government postal service has a much higher threshhold, thus it ironically may not be the best option to go with a private courier, especially if your seller asks for your choice.   But just remember, the cut-offs vary, and INCLUDE your TOTAL postage costs and are based on the declared contents value, and NOT the purchase price of the item, plus the inspection fee, if combined these totals exceed the cut-offs then you are charged the afore-mentioned fees.

Try to get into the habit of never bidding on an item until you have asked the seller for postage charges to your country and all charges and what courier they use, so that you don't receive an unexpected bill from them.  Its best to wait for their reply to see what the total for postage will be, and then you can factor that into your maximum bid price, so you don't find yourself out of pocket.

Its best if the item you want is being sold from a few sellers to watch a few beforehand and try to acquire a rough idea as to what the product is approximately worth.

Try as best as you can to research the seller, and its always best to nominate for insurance when purchasing from overseas, so you can track the parcel or at least are insured in case of theft or damage, and try to pay with a safe paying medium such as Paypal.  Its also good practice to purchase from highly recommended sellers, in other words, sellers with high feedback ratings with VERY to almost no bad feedback, and if possible, try to minimize your purchases to sellers who ship here to Australia constantly, as they will have a better understanding of charges, and give you a good indication of what to expect.

The benefits of buying from overseas are obvious in that there are a lot more products to choose from, but just remember some items are not allowed into Australia as restricted from Quarantine and Customs, so just be sure what you are buying is allowed.  But at the end of the day, if you can find the item you want in the same city as you, its almost always recommended to purchase it locally, so long as you feel comfortable about the seller.

Have fun and may your ebay experience be a wonderful opportunity and pleasure for you.