If it is only needler cartridges that are having this problem then make them have different prices at different merchants.
Needler cartriges = ammo. Every SG-vendor will buy it for base price.
You can craft them fast.
You can sell them in big quantity, because of low weight.
So you can "craft" your caps.
But this topic isn't about this.
This topic is about why do one vendors have base price as X, other as Y, 3rd as Z, etc. When you can find atleast one vendor who is interested in X+Y, and then if X have no caps, but Y have alot, you can sell to X your X-items and buy Y-items. Then sell Y-items to Y. So it's the same if you sell X-items to Y.
That's why selling/buying prices must be different, of couse in benefit of buying.
Edit:
Ahahahaha, he always got caps, after my post there're like 5-6 players, he don't have caps anymore, only tones of leather jackets, combat LJ, leather mk1-2, etc. ^_^ ^_^ ^_^
Hub became alive for some time, till Jake (or Jacob, or what's his name) lost all his caps for armor from gunrunners or other place.
Really I never saw so much people at Hub at once.