I'm well aware of a charge for GST but I don't believe the GST charge is shown at Ebays will.  

They should ONLY show a price which includes GST no matter wher the item comes from. Any item inported into Australia is subject to GST unless that item has an exemption.

An eBay member can nominate ANY delivery address, including one in the seller's country. Would you be happy to pay GST on a purchase going from the US to the US? If you were getting your relative there to onforward it to Oz, you would be paying double GST. Okay with that?

eBay can only know where the item is being delivered after a delivery address is provided.

The law regarding GST for LVI, is different to that for Domestic GST.

GST does not have to be included in an international listed sale price, and can be added to the sale price at time of sale.

Note,  GST  for HVI (over $1000)  remains unchanged and is applied and collected at the border.

But why does eBay send me an offer in AU$ but if I then want to purchase it, GST is added. Why don't they post the price inclusive og GST?

Why don't you ask eBay?

You asked if other members know, and they have explained it to you but you don't like their (factual) replies 

Contact eBay and then let us know what scripted response they give you

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@michaelb8892 wrote:

It was a simple question but the responses were not simple.

 

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eBay CS reps will give you a simple response - though it could well be irrelevant......or more complicated than the responses here.