I want to add that I was also extremely disappointed to find that he is gone after the quest. I did not purchase anyof his exclusive legendary gear at the time, because like all other shops I assumed he would remain in place. His stand is still there after the quest but he is just gone. I feel as if this must be an oversight - why would he just leave?
He does leave, never to return. Sometimes people just do that. It is hard to say why.
My gut feel is that this is another dev decision. At some point, the quest designers decided that the questing part of this game was not really open world. Once you get that Quest Complete, they lock all the doors, despawn or mute NPCs, and generally expect you to never come back. Story complete. Thanks for playing. Rolling credits, now. Like with action-adventure shooters, you need to loot the level (quest area) before you leave, or you will have to load a save game, or play another game, if you want any loot you missed.
Of course, on PC, you can just cheat and get back into the area. Sometimes, they just lock the doors and the loot is still there. In the case of this guy, you have to cheat to spawn the item you missed.
