I just reached the hub, and went directly to the gun store.

I have around 40% of barter, and noticed the NPC was selling knives for 38, and I could sell them for 40...

But I remember seeing other prices somewhere else...

Do shop prices vary according to NPCs?

How barter work?

If prices vary, what NPCs are good to exploit? (ie: buy their stuff to sell it back)

I've found NPCs consistent in their prices. Even if my Barter skill is different, Person A will always try to charge more than Person B. I think your standing with a merchant's particular faction, (perhaps city), influences prices, too.
Also, if you have Sex Appeal, opposite gender NPCs will give you better deals and same gender NPCs will give you worse deals.

Heh, barter is very broken in Fallout 1...

I ran around upping barter and then scamming NPCs out of their entire stocks of whatever I wanted =D

One of the mods I got (mostly to fix broken stuff, it was not intended to change anything) made one NPC sell outrageous amounts of books, I tested the barter bugs on that NPC, and now I am lugging around 600 copies of Big Book of Science =D

Yeah, makes no sense... but it is rather cool!

Even if you never put a single point in barter the game is so loot-heavy you should be swimming in money well before the end.