After a really short time of playing any common or uncommon equipment become useless and will always be traded. The problem is you do not know either the element nor the ability when you upgrade these items to the rare state. And instead of investing your resources into something, that might result in the desired element, it is better to wait until you get it as rare element and then you can decide if you like the item or not.
There should be a possibility to know what you get when you upgrade stuff to the rare state. The abilities of equipment could be included into the overview accessible in the dungeons or when you upgrade it to rare. The element bonus could be provided by either giving uncommon equipment a +1 bonus or it should also be available in the upgrade overview when you move it to rare.
In my case of my about 50 rare items I got, I only got two for blue bonus. With the Necromancer being my primary class this makes me little bit upset, because it feels so random. Allowing me to see what all the uncommon stuff would become, it does not feel so random anymore.