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"I am selling them per kilogram, they're pretty light so you can get around ten for a gold coin. How about that?" Asked the Mountain Giant.
A gold coin was able to feed a commoner family for a month, but for expensive merchandise like this, only ten were worth that, even if these were actually cheaper than they are originally. He probably is reselling them of course, that's how he makes money and how merchants' profit from traveling around, they buy and sell, they never just magically find things to sell, they do investments and find places where they're rare and sell them off. They're part of how societies advanced into modernity in Earth, as they were able to connect people and cultures all for the same sake… profits! …And surviving, I guess.
Check "The Protagnist's Sister Is Actually the Strongest"!