I think he should sell everything. All materials, shards, energies, Radiant and Ascendant, Motes, Coins, Orbs, Cores, all manner of consumables... The works. He should keep selling exotics like he does right along, but have the added function of being a sort of currency exchange that stops in every week. 90 Ascendant Shards, but no Energies? No problem. For Some Glimmer Xûr will happily change what you have into what you want. Of course, the exchange rates shouldn't be equal; Your 90 shards should only make 60 Energies, and vice-versa. And everything should have the prices hiked, since specific currencies will be easier to obtain, but at least we all wouldn't constantly get screwed by RNG.