Get Quick Caps (No DLCs Needed)

• A high Barter[fallout.wiki] level
  
• A decent Lockpicking[fallout.wiki] level
  
• A decent carry weight[fallout.wiki]

(This can still work with a low Barter but it being on the high end would be ideal)

Get to a point in the game where you can travel to the Gunrunners vendor.

Once you've gotten there you need to lockpick the gate.

Now that you're inside the gated area you need to kill the two guards patrolling the exterior of the building.

Loot the weapons and armour on their corpses and then sell them to the Vendortron.
They should be near perfect condition which'll make them worth a lot of caps each.
(This is why a high Barter level is ideal because it determines your sell price)

Now enter the building.
Inside there should be at least two armed guards patrolling the interior.
Kill them and repeat the fourth step.

Return to the interior and go to the backmost area.
Inside should be a few people making guns.
Kill them and take any weapons you find inside. (They should be in perfect condition)
Repeat the latter half of the fourth step and you should now have a pretty sizable amount of caps.

If you go outside the building and wait a few in game days at least one of the guards inside will be replaced/resurrected, allowing you to kill them again and repeat the previous process. The Vendortron will be restocked with caps every in game week or so.

As far as I've experienced this can be repeated infinitely.
With patience you'll be rich in no time.