I don't know much about this but I can say what I remember.
When you build an Air transport Corp, it uses an Airport instead of a production plant.
Historically, airports were only used for this purpose and were ordered automatically when you requested the building of an Air transport corporation. So they were never available for countries to buy.
Now they are used by Assets maintenance corps. Obviously, the above situation was forgotten by the time Assets maintenance was introduced. On the surface of it, it seems to not matter.
However if there are shortages of airports and we need to supply them to our Assets main corps, it would be nice to store a supply in the country and sell them to the corps. However I imagine that once airports are purchased by a country, they cannot be resold?
The GM should do some thinking around this, for sure.
-hymy, since 2001-