Magical weapons have little use to merchants as well except as sales to people who want them. Same goes for soul coins. They're not just valuable to fiends, but also people who deal with fiends such as magi.
If the game economy at all respected opportunity, supply, and demand one might make an argument that the merchants we have in-game would not pay much for coins because they don't have any wizards nearby who would buy them in turn. But then they also shouldn't buy whatever spare loot we throw at them that they alsodon't have any salesdemand for. Since that's not true for any other item, it shouldn't be for soul coins either.
