As far as I know. There's one exotic armor piece for each class he can't sell. I believe (I certainly could be wrong) that Praxic is the warlock piece he will never sell. Like the Insurmountable effort. And something else.

edit: maybe that's the reason. But warlock hasn't had 2 armor pieces so far. Skull of dire ahamkara and praxic fire chest

I never said it was one of the Chests for every class. There's ONE item that he doesn't have listed in his database of possible items over at DestinyDB. In the case of the warlock. He doesn't have listed the chest Heart of The Praxic Fire. But all of that its speculation too. He could sell it eventually; who knows :)

Voidfang Vestments for the eleventy-millionth time. Next week is sunbreakers t.t