The armorer near WBB buys them but I can't seem to find any other who do. I've only checked those in brit and skara currently.
Funnily enough this was reported by another player just a couple of days ago. I've already fixed it, with the next patch all armorers will buy all types of armor.
At the moment it is random what they buy/sell list is for armorers. Some will buy/sell leather, others studded, etc.
Uhm, leather armor istraded by tanners in aleather workers shop. Armorers is working with metal in a armory.
But who knows The "Heavy Metal Armorer" shopin Britain might want to hit the leather market