In the quart-size resealable bag, combine sugar and buttermilk. Seal the bag. In the gallon size resealable bag, combine salt and ice. Insert the smaller bag into the larger one and seal, letting all the air out. Shake continuously for 5 minutes. When buttermilk stiffens, remove the small bag and dip into cold water to remove salt from the outside of the bag. Freeze or serve immediately. To serve, open bag and scoop out sorbet.