Huge Noob Question: How does game trading/selling work?

[Moren]

d21lewis said:
Pikmin 2 is expensive? I got it for $5.....I never considered trading it in but you've got my attention.

http://www.amazon.com/Pikmin-2-GameCube/dp/B00009WAUE/ref=sr_1_1?ie=UTF8&qid=1307841929&sr=8-1

Just so you have an idea -_-