Nineteen-year-old Zippy can hardly believe it: she’s the newest and youngest salesgirl at I. Magnin, “San Francisco’s Finest Department Store.” Every week, she rotates her three spruced-up Salvation Army outfits and Vaseline-shined pumps;still, she’s thrilled to walk those pumps through the employee entrance five days a week as she saves to buy something new. For a girl who grew up in a one-bedroom apartment above a liquor store with her mother and her mother’s madcap boyfriend, Howard;a girl who wanted to go to college but had no help in figuring out how;I. Magnin represents a real chance for a better and more elegant life. Or, at the very least, a more interesting one. Zippy may not be in school, but she’s about to get an education that will stick with her for decades. Her fellow salesgirls (lifetime professionals) run the gamut from mean and indifferent to caring and helpful. The cosmetics ladies on the first floor share both samples and advice (“only date a man with a Rolex”);and h