It is getting a nerfed in a few weeks, so buying it now is a waste anyway.
Edit: And for all those who have bought Gjallahorn today and say 'The Nerf is not that bad' think again. Why would xur sell the current best gun in the game weeks before it's Nerf? Because after the Nerf it will be trash. Bungie is giving all the none-Gjally owners a chance to use it in its full glory.
English
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Dumb logic
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This is the second time Xur has sold it. There may also be a new gun that will outshine the Gjallahorn in The Taken King. Who knows?