After finishing The Human Error quest with the resolution that made it impossible to buy the unique equipment from Penny Fitzgerald (Destroyer's Helmet and Justice), I realized there must be unique gear that can become unobtainable after certain quests/events if not previously obtained.
I also know there is a point later in the main story quest line that will make you an enemy of some factions, which probably leads to more unobtainable gear if they aren't purchased/obtained before the respective quest is complete.
Is there a list of gear that can become unobtainable, and the quest/event that makes it unobtainable? I'd hate to find out down the line that I missed desirable unique equipment by unknowingly progressing through a quest.
Best Answer
There are a variety of weapons available from both side quests and faction quests, depending on your allegiances in the game and how thoroughly you clear out the side quests it's quite easy to miss many. This list doesn't include weapons you may find as a result of murdering friendlies (for example killing Virgil to get Virgil's rifle).
Side quests
Below is the list of items directly available as a result of quests:
In addition to the above list, there are a variety of instances throughout the game where as a result of your actions a NPC vendor will become hostile to you. Such examples include the Battle of Bunker Hill resulting in the deaths of vendor NPCs like Deb, and Human Error resulting in the citizens of Covenant becoming hostile you, like Penny Fitzgerald. I'm not going to include Diamond City or Goodneighbor in this list because if you become hostile to them that's your own fault!
Sold by Deb in Bunker Hill
Sold by Penny in Covenant
Brotherhood of Steel quests
With the exception of Righteous Authority, all of the weapons below require progression through the Brotherhood of Steel quest line to the point where you have access to the Prydwen, and then some more require you to hold a particular rank within the Brotherhood.
Needless to say, if you become hostile to the Brotherhood of Steel you will lose access to this faction's vendors.
Requisitioned from Teagan
Minutemen quests
The Minutemen don't really have anything all that special, certainly not before taking back the Castle but then you do get access to a Gauss Rifle...
Railroad quests
I haven't included the Railway Rifle in this list as it is possible to find this elsewhere, but the Railway Rifle can be a possible quest reward for completing Underground Undercover. Failing that you can head over to Big John's Salvage for a guaranteed spawn.
Needless to say, if you become hostile to the Railroad you will lose access to this faction's vendors.
Institute quests
The super powerful Experiment 18A is available very early on in the Institute quest line, you can pick it up immediately after speaking to each of the department heads before you first leave the Institute. The Wazer Wifle is an end game weapon only available after completing three unmarked quests to fetch items from the wasteland.
Needless to say, if you become hostile to the Institute you will lose access to this faction's vendors.
From Institute requisitions