I believe it is entirely random.
I play Borderlands far too much, and I've never found any pattern to it.
It's definitely worth it to check whenever you're in New Haven, as the hidden vendor has a higher than normal chance of having high quality weapons.
I tend to find a lot of oranges in it.
gaming.stackexchange.com/users/15092/SaintWacko
You must have completed "Hair of the Dog" from Earl.
After that it should be open all the time, the fact that it isn't appears to be a bug .
I found that when playing a multiplayer game the door was always open.
In single player, teleporting to New Haven from Middle of Nowhere seemed to almost always open the door.
gaming.stackexchange.com/users/20818/Kexlox
If you don't mind a bit of tweaking, you can access the shop through the wall even if its closed as long as you've enabled the vendor by visiting the "Middle of Nowhere" waypoint.
c/p from the video comments: to enable 3rd person view in Borderlands
here's what You have to do:
Go to:
C:\Documents and Settings\Username\documents\My
Games\Borderlands\WillowGame\Config Open:
WillowInput.ini Search for [WillowGame.
WillowPlayerInput] Add the following below other binds: Bindings=(Name="F3",Command="Camera
ThirdPerson",Control=False,Shift=
False,Alt
=False)
Bindings=(Name="F4",Command="Camera FirstPerson",Control=
False,Shift=
False,Alt
=False)
Now If You press F3 You will go to third person view, F4 brings back first person view, You can change the keys to others I simply prefer using this,
gaming.stackexchange.com/users/3917/l I
From my experience, its only open if you are doing the quest line from Marcus & you are visiting New Haven during the daytime.
I also found this:
This opened once I completed the quest 'IVE GOT A SINKING FEELING' and found the 110101 Eridian Blaster.
gaming.stackexchange.com/users/18818/Zero
It's usually stuck closed if you start off in New Haven that play session.
Meaning, if you log out in New Haven to farm the chests when you get back on, checking the hidden vendor along with it doesn't quite go to plan as the door will usually be closed.
Porting out of town and back should open it back up.
It does only open for the first time after you've reached far enough in the game.
I believe reaching Middle of Nowhere unlocks it as others mentioned.
gaming.stackexchange.com/users/177835/desolation0
I found if you log out anywhere BUT new haven and log back in, then FT back to new haven, it will be open.
Tried it 10 times, worked 10 times.
So, don't log out in New Haven or the door will be locked the whole play session.
Did this on PC by the way.
Not sure if it will work on xbox360 or not.
Happy Vault Hunting! EDIT
- I can also confirm this vendor box will only open up when you get to Middle of Nowhere.
gaming.stackexchange.com/users/215526/BlueStraza
In New Haven, there is sometimes a second weapons vendor across the street from the usual one.  However, it isn't  always  open.  Is there anything I can do to ensure it is available?
How do I gain access to the hidden weapons vendor in New Haven?
gaming.stackexchange.com
<borderlands>
gaming.stackexchange.com/questions/57368
gaming.stackexchange.com/users/None/
It's only open if you are doing the quest line from Marcus and visiting New Haven. You must have completed Hair of the Dog. There are few steps that need to be followed before accessing the vendor in question.
The community is offering some suggestions.