The game is broken to the level where supply and demand doesn't exist in this game. You could just plop a R 100% zoning without any city in your region as well.
It's pretty easy to do it XL including making a tunnel under the sea to the next island etc... I started to wonder now if the Cities XL is more flexible in size, farming, does more calculation and computing than the Sim city itself without the stupid agents...
Because if you have only 1 trade depot, only 1 type of the resource export truck will come. The way to fix this is to build another trade depot to dedicate the specific resource itself. Seems like your coal and ore are selling too well over the oil.