Everybody knows you can buy things duty free at airports if you are leaving for or coming from another country.
(You can also buy on airplanes and I think I've seen on some international ferries and some land border crossings.)
But rent for retail space in airports is so high that you can often end up paying higher prices without duty at the airport than you could pay including the duty in a regular shop in the city away from the airport. I guess this is why airport duty free shops are full of luxury items...
But I seem to recall being told the best way to shop when travelling overseas is to do something like buying from duty free shops in cities or somehow buying from regular shops but without duty ... but I don't know the details.
Where can you buy things duty-free outside airports? Can any shop sell without duty to people with plane tickets? Are there duty-free shops in most cities? If it varies by country and city, what are the common variations and how can I know which is possible in a given place?