A nice addition to the mat trade window would be individual indications of how full up the player currently is, for each material (ie. 45/250). Often, I'm not looking for any one material, but rather balancing out my stocks so that none are completely full, if I run across more to pick up. An indicator would inform me better what to trade away. I cobbled together an example from one of the trader windows on the mat trader screen,
as an example of the additional information in the right top corner.