When selecting sweet onions of any variety in the store, there are a few things to consider. Sweet onions are fresher than regular onions and are more susceptible to bruising. Look for firm dry onions with shiny skins. Check the root end of the bulb for any signs of breakdown or decay such as gray mold. On the top of the onion, check for moisture and softness by pressing in with the thumb and forefinger right near the stem. It should be firm and dry.
At home store them in a cool dry airy location. These onions are high in water and sugar content so avoid bruising and keep them away form tubers or other dry vegetables. If you are planning to store them for a lengthy period of time then check them periodically for break down. Cut onions should be wrapped in plastic and stored in the refrigerator for up to a week.

In Season Now: Vidalia Sweet Onions