Answer:
I have roots nobody sees. I am taller than trees. Up up I go but I never grow.
When you buy me I am costly, but the only use I have is hanging only. What am I?
I am a mini solar powered computer
I am shared by goats devils and unicorns.
Marking mortal privation when firmly in place. An enduring summation inscribed in my face.
No matter how little or how much you use me, you change me every month.