Some duty free stores in the US are now selling to all passengers (but non-international travelers will pay regular taxes on the items). I don't know what the specific situation is in Atlanta (although according to the Atlanta airport's web site, the Duty Free store is only for international passengers).

That being said, since you're not allowed to consume alcohol that you bring on board yourself (well, technically, you can consume it if a FA serves it to you, but I'm not aware of any US carriers that normally allow their FAs to serve alcohol that isn't supplied by the airline), I'm not sure what you are looking to accomplish. While it may not be the cheapest solution, your best bet is to bring your credit card and buy it on-board.