Hah! The only thing I ever used them for was to sell cigarettes. No way you'd be screwed without them, because they never sell anything of interest.
Actually, that's true.
Here's what you need:
For Explosives, go buy a GMG at Mick And Ralphs, then kill them.
For Guns or Explosives use Gun Runners exclusively.
For EW use Van Graffs exclusively.
Melee and Unarmed doesn't need anything at all, maybe that Major Knight at Mojave Outpost to repair stuff but otherwise you can kill every single merchant.
Every other merchant in the game can be killed and looted without much problem.
But leave "one" doctor alive.