The ad should say "AU model" or similar if it's an Australian phone.
My son got done like that. He bought a Samsung (not sure what model) new from an Ebay seller. 3 months later there was a bright green line down the screen.
I tried to get it fixed under warranty by Samsung Australia, they didn't want to know about it because it was a US model.
Contacted Samsung US, they didn't want to know me as I wasn't the registered purchaser - the arsehole selling them on Ebay was.
Contacted the seller - no response.
Basically he could have taken the phone back and as the registered US buyer, got it replaced under warranty. But he couldn't be bothered.
Contacted Ebay and put in a complaint, but they wouldn't even let me change my rating, so he's probably still flogging US stock.
So my son is stuck with the line on the screen, because it would cost about $350 to replace the screen and the phone was only $400.