Bargaining can be difficult if you live in a culture where it's not the norm. Especially when the only exposure you get to it is on trips to different countries, and those different places have their own unwritten rules and expectations built into the process.

For example, in various places in Asia, if you pay more than about half the initial asking price for anything, you're essentially being fleeced. When you ask how much something is, if they say 100, you're expected to offer them 20 as a starting point, and the to and fro process of bargaining is underway.

If you've learnt that and then you travel to, say, Peru, you might come across someone asking 100 for something. If you offer 20 in response, the reaction will either be that you have amused them beyond belief, or that you have insulted them beyond belief. You better hope they're in a good mood. Because there, if you bargain someone down to 10% off, you've done a good job.*

Roleplaying games tend to use bargaining a bit, but it's kind of expected that PCs buy goods at a set price as given in the rulebook equipment lists, and if PCs try to sell stuff, they pretty much know they can expect to get about half that price. Which makes the bargaining a bit perfunctory. Instead, try mixing it up a bit. Have the unwritten bargaining protocols change drastically from country to country, or even from town to town. Throw in some really unusual stuff, like the expectation that the buyer must also buy the seller a drink to seal the deal. Or that the first offered buying price must be deliberately over what the seller expects to get, so that the seller can make a point of how miserable an item it is, and in his humble opinion it's not worth nearly that much, perhaps the buyer would care to offer less?

Of course, if a foreigner is ignorant of these traditions, things can (and should) go awry. Doubly of course, PCs should be ignorant of these traditions until they need to buy or sell something...

* This writer speaks from harsh experience...

Transcript

Wioslea: {an alien} I'll give you two thousand.
Luke: But it's worth 4000!
Wioslea: 2000.
Luke: It's practically new, look at it. 3900.
Wioslea: 2000.
Luke: What are you trying to pull here? Okay, okay, I'll be generous, we're in a hurry. 3500.
Wioslea: 2000.
Obi-Wan: Take it.
Luke: But that's half its value!
Obi-Wan: You buy stuff at full price, you sell at half.
R2-D2: First Law of Equipment Trading.
Luke: So all these traders out there are making 100% profit on every transaction?
Luke: You'd think they'd have banded together and taken over the Galaxy by now.
R2-D2: What, kind of like some sort of... Trade Federation?