You got it.
It's my understanding that the 1 million credit trade limit is based on the car's price/value if it has zero mileage/kilometerage.
(Off topic question, for the majority of the world that uses in kilometers, do you reference the accumulated distance a car has traveled as "mileage", because I don't believe "kilometerage" is a real word, and I don't know the word that would apply...)
If you can buy a used car for close to under 1 million credits, odds are that it can't be gifted/traded because the same car without being driven would pop over the threshold.
You can always go to one of the online GT5 car database websites for reference whether a can can or cannot be traded. (I know that
www.mygranturismo.net has this data available.)
Hope this helps!
Peace...