Upvote:4
Let's say your return flight is from Atlanta to Seattle, and Seattle to Vancouver.
That first flight is going to be a domestic flight. Thus, in Atlanta, you won't have access to a duty-free shop as you won't have gone in an "international zone" just yet.
Then, in Seattle, I'm guessing you're going to transfer from the domestic zone to the international zone, at which point you should be able to.
Upvote:4
Duty-free sales are restricted to international travellers. Because your boarding pass in Atlanta will be for a domestic flight, even if you're passing by a DF shop (as international departures in American airports are usually not segregated), you won't be able to buy in Atlanta, even if you have another boarding pass for your next flight.
Were you allowed to buy some booze in Atlanta, you could offload it in Seattle for example, and give it to someone. So that's a no-no: you can only buy DF items when you're leaving the country, ie at your last departure point.
Also, I don't remember how it works in the USA, but here in Asia, you can't buy liquids in DF stores if you have a transfer (even international), as they would get caught during security checks at the transfer airport. One would think that TSA would object too, especially considering the non-segregated terminals.