So I was just browsing the new Armory section on Bungie.net and noticed that for Thorn it says that "Xur, Agent of the Nine, sometimes sells this item in the Tower". I owned this game since Beta and I have never seen Xur sell Thorn or any exotic bounty weapons. As far as I was aware, Xur will not sell weapons that you attain from exotic bounties. Is this is a mistake or has Bungie changed RNG for Xur to include exotic bounty weapons.
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They most likely did not update his inventory. He used to sell upgrades for those weapons. If you got a gjallarhorn or a bounty weapon or PS exclusive before the dark below the only way you can upgrade was through xur to the higher attack values. Now that we can ascend exotics there is no need for him to sell the upgrades anymore.