Does buying with skulls compete with story unlocks?
Solution 1:
You can't buy any traps or weapons that are naturally unlocked through the story with skulls. You can see what is and isn't available for purchase in your spellbook. Where most traps have the number of skulls needed to purchase them, story unlocks have a little shield that will show what is needed to unlock them upon mouseover.
Your friend is either lying or he's mis-remembering something. Perhaps he purchased an upgrade for them, seeing as how they are unlocked very early in the story.