I can see pictures in your listing of the power supply box in one it says EU UK  on the side the other pic has AU on the side - I cant see any that have US on the side...

That being said - and i hate to play 'devil's advocate' but if you're going to sell electrical items internationally you should state input voltage in your listing (my opinion), and be aware of other countries laws regarding the sale of such.

On the other hand if they had found it on ebay au, and you did not have worldwide postage but they pestered you to send it o/s, I would have said it was their responsibility to check.

Sorry if its not what you want to hear - just my opinion - i dont mean to insult or offend 🙂