Create a character with good barter skills, and upgrade them whenever you level up. When bartering with someone, you can easily trade back and forth the same item. Empty them of all their items if you desire, leaving them with 1 cap. This works with everyone with a lower barter skill than you. The best person to do this with is the dwarven man with the gunrunners, especially considering the "Weight glitch". Basically, you can say to someone: "Swap my stimpak for your identical stimpak and all your money." It takes a while, but you can leave the gunrunner trader with just his (regenerating) caps, and keep swapping items around to get more.