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 (the same price that you would get if you sell the same good to a merchant, but are open end 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: