I agree with the advice to contact the manufacturer and find out what temperature range they specify for the freezer. While colder temperatures will increase the efficiency of the condenser in getting rid of the heat removed from the food, the oil in the compressor is going to get more viscous with colder temperatures and may not lubricate the moving parts in the compressor sufficiently.
Chest freezers are inherently more efficient than upright freezers because cold air is denser than warm air. So, when you open a chest freezer, the cold air remains inside as Derby Dad says.
Never ever never buy a used chest freezer if it's been turned off. That's because chest freezers are built differently than refrigerators. In a chest type freezer the evaporator coils line the inside of the freezer side walls and the condenser coils line the outsides of the freezer side walls. That's why the walls of a chest freezer will feel warm, just like the condenser coils on the back of a fridge when it's running. Because chest freezers don't have a automatic defrost like fridges, over the course of several years, frost will accumulate on the evaporator coils of the freezer. This space is sealed off and so you don't get the kind of frost accumulation that you get on a manual defrost fridge, but it will accumulate with time.
The problem is is that space is sealed off. So, if you turn off or unplug the freezer, that frost melts and forms a puddle at the bottom of the freezer. That water won't do any harm at all to the aluminum evaporator coils on the inside of the freezer walls, but in time it will cause corrosion of the STEEL condensor coils on the outside of the freezer walls. That corrosion will make the steel rough, and that results in what engineers call "stress raisers". If you have a notch in a steel tube, and you continuously pressurize and depressurize that tube, a crack will propogate from that notch because the stress on the steel is highest at the notch because the wall thickness of the tubing is smallest at the notch.
So, if you plan to sell an old freezer, put your ad in the paper, but leave the freezer plugged in and running until it's sold. Otherwise, knowledgable buyers won't want to buy it for fear it's not going to last long.