iPhone duty free purchase

If you are buying the phone in the USA there is no export duty, but there probably is an import duty at whatever your final destination if you declare the purchase upon arrival (or if you don’t declare it but get caught). For purchases of anything in the US (including phones) there is sales tax. There is no way to avoid paying sales tax and no way to get a refund of sales tax. The rate will vary by where you buy, however. A couple of states have no sales tax (Montana and Delaware come to mind), but most localities in the US will have a tax, typically between 5% and 12%. As the sales tax can include State, County and Local sales tax, there are roughly 6,000 different taxing authorities in the US, with different rates, usually in that range. (There is no national sales tax, however.) For example, New York City sales tax is 8%, San Francisco is 8.63%, and Seattle is 10.25%.

I suppose that depends on where you are...

I sincerely doubt you can purchase one from an authorized retailer.

At best you would be buying a used or gray market device.