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A trader, unlike a merchant, does not sell and buy goods at a fixed price, but depending on supply and demand.
You may even find a trader "out of stock" if there is very little supply.
Prices have a lower limit (minimum purchase prices are 3 gold for common materials, 30 gold per unit for rare materials and 25 gold for Runes and Insignias, minimum sale prices are 10 gold for common materials, 100 gold for rare materials and 100 gold for Runes and Insignia, the minimum price for Celestial Sigils is unknown), but are open ended to an extent. Prices for certain Superior Runes have gone up to 100 platinum at one point, but since the maximum amount of gold you can carry on your person is 100 platinum, this is essentially upper limit.
These are the traders in the game: