in an ideal situation - the resources are random, and therefore, everyone's making enough to share. By end of Iron, everyone should be well established with their bronze resources, and trading should be fairly frequent. I often have several trades posted. If all the players around you are active, then there should be lots of everything.
My concern is that in my local trade block, it appears things aren't in fact random. There are 60 trades posted, 41 of them offer dye or marble. Considering those are my resources too, I'm wondering if the game's randomizer needs a check.