NPCs do not have to follow the same rules PCs do and can use different prices.
Why does that merchant sell goods to you at listed price? Because he got them cheaper. He is part of a trading network supplying iron from dwarves to the lowlands.
Why does that merchant buy goods from you at the listed price? Because he knows where to sell them for a comfortable markup. There is an island nation without any sources of iron. They will pay more for it.
Why does that shop buy wood at listed price? Eh, it is the market price, if not from you, they would buy it at the same price from a shop down the street.
Why does that shop sell wood at that price? They bought it for lower price. If you do an advance payment for a certain amount of wood before cutting even starts, you can get it cheaper, you know.
Why can't the party do that?
Because they are not merchants. Dwarves can sell you iron...for a nominal price. Sorry, their bulk discounts start from 100'000 gold pieces spent per year. That island nation? Nominal price. You are not member of the XXX nation merchant guild, right? Inflated prices are actually an informal way of paying off that country so it will not be overly aggressive in military, economical or political dealings. Woodcutters? Sure, pay now, be ready to accept all wood in 2-3 month. If you are not ready the moment we are back in town, deal is off and you will have to pay the difference with market price, if you still want your wood.
Business is a time-consuming practice, which is ill-suited for adventurers. Unless your game is based on running a business or trading, it is better to let traders get better deals in background, away from PC path and leave players with knowledge that they can convert 10m³ of accumulated gold into bag of gems and back without losing a penny.
