If the buyer lives in Australia ebay will charge them (the buyer) 10% GST (goods and services tax) . The Australian government brought this in about 18 months ago. Hannants and Amazon, along with a few other bigger retailers do. They (the retailers) collect the tax and return it to the Australian government.
There are "rules" that outline who is supposed to collect the tax (one being that the retailer had to do over a set amount of business with Australia customes0 , but it was always unclear how this would be enforced. Most smaller retailers don't have to. Amazon for a while geoblocked Australians from the Amazon.com site saying it was to difficult to collect. Something we all thought was strange as they seem to charge different taxes depending on the state in the US you are. They directed us to the .com.au site which has less choice and is often more expensive (even with shipping) than the .com site. This has been now been changed (although for many things there is still a local company that is much cheaper than Amazon.)
As an aside the reason most people in Australia don't buy from the US anymore is the postage rates. They are very expensive compared to Europe and the far east.