Normally one should not sell the freezer and use the refrigerator or vice versa. Certainly one cannot do so for different shelves of the fridge. The best solution is to ask a neighbor who is selling their freezer (or refrigerator) to place your chometz there (and remember to add that location when you sell your chometz).

If there are difficult circumstances, one may sell the entire freezer section or the entire refrigerator section, and tape it shut and mark it as “chometz.”