Consider this hypothetical situation:  Your best friend, a psychology major approaches you on campus.  (see why this is hypothetical, why would you be friends with a psychology major?
Your friend, who is confused and needs your help, says "I just started working at a used car dealership.  My commission will be based on the gross margin of each sale.  What does that mean?  And how can I maximize my commission checks?"

What advice would you give to your friend (including an explanation of the concept of gross margin)?