I am an Australian accountant with many years experience in the plant & machinery auction business, so I have had plenty of experience with the financial side of the export of second-hand goods. If you buy from a registered business in Australia they are obliged to charge you 10% GST no matter where in the world you are. If you can provide written evidence that the goods have actually been exported within 60 days after the sale you can claim a refund of the GST from the Australian Taxation Office (ATO). Some businesses, such as ours was, have a private ruling from the ATO that they (the business) can retain the GST until the 60-day period has expired and can refund the GST to the buyer within that period if the buyer produces satisfactory evidence of export. This makes it easier for the buyer. I am afraid I have no knowledge of what might befall you at the US end with regard to taxes and duties.