The AI never trades cities! if it was of my 500 Huge cities pack with all resources, for just on puny little city of there with no resources they wont do it!
They only time ive got it is when I was killing them they would give up all there cities
The AI parts with cities only in peace negotiations. If you offer a city of yours, it will not accept the deal unless you give the city for free. It's done to prevent the human from cheating the AI, for instance to trade a border city for a tech and instantly retake it.
eerrr makes sense but it would be cool to have it so there would have to be a special treaty to be signed to trade citites and if broken between the time (20 turns?) MASS distrust happends but not to the exterme of ALL civs declaring war