Answer:
Neither bought nor sold but more valuable than gold. It is built but not by hand. What is it?
Though easy to spot, when allowed to plume, It is hard to see, when held in a room.
Mountains will crumble and temples will fall. And no man can survive its endless call.
The more you take, the more you leave behind.
Take off my skin, I won't cry, but you will.
You use it between your head and your toes, the more it works the thinner it grows.
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?