I asked a guy in PC world UK and he said they get up to 15% commission on what they sell!!! I was blown away by that, presumed Apple sales people would be on a similar level.
That guy's talking crap. PC World staff don't get commission. They operate on a "don't sell enough warranties and you're on probation, still don't sell enough and you're fired" policy. I've been there, done that (fairly recently).
There are bonuses if you make over a certain amount of profit, but that involves selling at least three times what the average person would, everyone getting a warranty, and getting a few Sky TV or mobile broadband signups every day.