Of course they can still sell perfume and model planes and the like, but not the booze and cigarettes where the big duty-free profits are to be made.)
AFAIK no inflight sales of any kind whether Duty Free or Perfumes etc will be allowed. The customs form is for listing all purchases being brought into the USA, not just Duty Free alcohol and cigarettes. But DF sales will be allowed on the flights FROM the US, so they won't quite disappear.
I confess I don't see a huge demand for this
There is a big demand for connecting flights in ORD and JFK. It would be of huge convenience for EI passengers of they were able to walk of their EI flight and straight to the gate of their next flight within the same terminal. Likewise CO in EWR and DL in ATL/JFK.