Well, either way, I think a lot of prices should be looked at and adjusted. My barter is 16%, and it costs $23 for a shotgun shell from Sha Enin in the NCR. $13 for a 10mm round. $13,000 for a combat shotgun/FN FAL. Nobody's going to buy that. I'll screenshot if you want.
If I figure right, base prices are the absolute minimum a player could possibly pay for that item, right? The base price isn't the maximum, so everyone actually pays a lot more.
As for trying to establish the role of NPC traders, I'd just say treat it as you would a business. We want people to be constantly spending money and exchanging goods. NPC traders should be the backbone of the economy, with players fulfilling the need for rarer goods.
An economy that works has cash and goods people want constantly changing hands. At the moment, that's not really what we have. Ideally, this cash and goods would be exchanged willingly. This is the problem. How do we get players to spend money?
There's a lot of answers to that. My own suggestion would be to make absolutely everything a whole lot cheaper. Little things like mercs and rooms and cars should also be sucking money out of players' pockets.
A few ideas related to that:
Mercs respawn. Now people are prepared to throw down $5,000 on a merc, because they won't lose them in one random encounter. After a while, the merc won't be strong enough for their needs and they'll upgrade. They also have to keep them supplied with guns and ammo.
Cars can be refueled for a price. You don't get the MFC cells. You just stop off at certain traders, like Smitty, T-Ray, Skeeter, and just ask for them to refuel your car. They do, and it costs you caps/barter. Cars themselves could do with being cheaper, I barely see any around these days.
Rentable rooms replace tents. It'll be unpopular, but players will have to pay for a safe place to store their stuff. Players ought to get the option to pay in either caps or barter. This'll require the whole renting system to be expanded, though, in terms of both maps and scripting.