The Heart of Praxic Fire and skull of dire Ahamkara and ATs/8 Arachnid are the only 3 exotic armor pieces never sold by Xur. Skull seems more common though to be dropped by RNG. I haven't seen a person with Heart on in months. Edit: destinyxur.com to see all items ever sold. Edit 2: if you got if from a chest engram that isn't what were talking about here so please clarify this to not confuse people to think it was actually sold. The upgrade also doesn't count as you must possess said item first Edit 3: T Roy finally pulled the "I was just trolling" card that all incorrect people pull when their feeble minds finally can't handle that their argument was wrong