I noticed that except normal items the NPCs also sell items of blue-named rarity as well. Is there a chance an NPC will sell items of higher rarity then blue-named? If so what is the highest possible rarity of an item you can find in the shop?
Merchants only sell white and blue items. The idea of selling rares is not implemented because there is already a fine chance of finding them while playing.
Also, there are orbs with which you can upgrade normal items to rare -so you can buy the base item you want in the store and upgrade it with the orb. Just trade for those orbs with other players if you don't find enough.