By Sunspot
#208613 I've had this same issue, too. It's always big nuggets given as quest rewards that seem to be bugged and refuse to be sold. They won't stack with other big nuggets, either. From my own experience, this is actually a common bug that seems to effect a majority of stackable quest rewards.

Gold ingots, diamonds, potions, almost anything I get from quests won't stack and won't sell in shops. However, if the bugged items are part of a crafting recipe, they will work normally when used in those recipes.