This leaked information was confirmed with Megamanx's data mining prediction streak being correct (including the nightfall).
Xur releases are a preset list, not random.
https://www.reddit.com/r/DestinyTheGame/comments/2xk9iz/misc_results_of_my_investigation_of_111spoilers/?sort=confidence
https://www.youtube.com/watch?v=YLxHKQA-Eew&feature=youtu.be
Gjallahorn will never be sold again so far because it was taken out of Xur's rotation, but Bungie can add it in manually as they can manipulate what Xur sells.
http://www.reddit.com/r/DestinyTheGame/comments/2wi7fq/misc_about_xur_sale_list_iron_banner_sale_list/
Xurs inventory this week; https://www.reddit.com/r/DestinyTheGame/comments/2xcqj7/discussion_xurs_stock_and_nightfall_mods_for_the/?sort=confidence
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Title is misleading, implying he isn't selling the first two aren't being sold this week.