Victor, thank you for asking. Let me tell you a few reasons for which the buyer may or may not want to purchase the said ipod.
Maybe the 30gig is in better condition?
Maybe the 30gig includes shipping price?
Maybe the 30gig has 3493409830958 songs?
Maybe the 30gig is white and he likes white better?
Maybe he is a normal person that doesn't have any use for 80gigs and doesn't want to waste the extra money?
F) All of the above