Haggling with traders is a time-honoured tradition in roleplaying games. The problem is that the laws of supply and demand get warped by the singular and unusual circumstances of a one-off transaction taking place without an effective larger economy.

Many packaged adventures state that in the specific village on the edge of danger where the adventures find themselves, prices are inflated and run at 2 to 10 times or more the standard prices listed in the rule books, and that the villagers won't lower their prices. This is a rough cut at the idea that adventuring supplies are more expensive where they are most needed, but the rigidity of the rule is unrealistically inflexible.

For any individual transaction, it's possible for the seller to hold out only as far as the buyer is willing to entertain the exchange. If you don't really need a new sword, there's no chance you'll pay 10 times the greater market value for one. On the other hand, if you've lost your weapon and absolutely must have a new one, you'll pay whatever it takes, and the seller - if aware of this situation - can hold out for much more.

But usually the seller isn't aware of how desperate the buyer is, so needs to set an initial price somewhat higher than what they would actually accept. And thus begins the dance of haggling. The lowest acceptable selling price is determined by outside market forces, of which traders would only be aware of inasmuch as they determine the general environment and gut feeling for the "fair price".

In short, it's fine to set a "fair price" that the traders are willing to accept, and have it be inflated due to low supply or high demand. But the traders should always aim higher, and be open to negotiation down to the fair price. And if they've had a poor day and need to make a sale, sometimes they can even go a bit lower. Just make sure to accompany it with appropriate lamentations about the trader's starving children.

Transcript

GM: The Ewocs start building fires beneath you.
Luke: What? Why?! I thought we were going to a seminar!
Logray: You ruined our smeerp trap. We have to eat something.
Han: Okay, fine. I'll trade you my blaster. You can shoot ten smeerps with it!
R2-D2: Yeah, because arming primitive native inhabitants has always worked well in the past.
[SFX]: < whir-boop fobuzz whroop blip-plip-ding bippity-bop-boop >
Logray: Allow me to consult the pricing models.
{Logray dances and rattles his staff}
[SFX]: dance... shake... dance... rattle... dance... shake...
Logray: Factoring in business risk, market hysteresis, information asymmetry, and vendor trust... forty-seven of these "blasters" would be an acceptable trade.
Han: I told you we should have kept all those blasters!