Make sure you buy mushrooms that are within their sell-by date. Generally the quality of button mushrooms does not deteriorate for 4-5 days after they are put on display. If the mushrooms are no longer snow-white, it does not mean they are less enjoyable. It simply indicates that they are near the end of their storage period and should be used soon.
Storing mushrooms at home
Always keep bought mushrooms in the fridge at between 2-8 °C until use. Mushroom trays are covered by a special micro-perforated foil which prevents mushrooms from drying out, reducing the speed of deterioration. So leave this layer on until you open the tray. If you buy loose mushrooms, it is worth putting them into a paper bag or onto a tray to keep them fresh in the fridge.