Answer:
the only amusing part of the body
I am black within and red without With four corners round about.
Can not be bought, Can not be sold, Even if it's made of gold. What is it?
You can build or destroy. Let creativity soar but be careful at night.
What goes up and down but does not move?
What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?