Make sure that all of the heating and cooling units in the house are working properly. Make sure that all of the filters are clean and the units are charged so that they work to their full potential. Don't leave any extra heating or cooling appliances in rooms, other than ceiling fans, while you are showing the home. It might suggest to a buyer that something is wrong with the regular units or that they can't adequately handle the capacity. Any kind of portable appliance that helps to heat or cool a room indicates a problem to a buyer. Make sure that medical equipment, dehumidifiers and ionizers are also out of sight. People do not want to see them when they are previewing a property.
If there is mold in the house, there may be a problem with humidity because some of the windows may not be sealed properly. Mold can be a real problem and can keep your home from selling especially if your buyer suffers from allergies. If you see that there is mold in the house, you can easily clean spot problems with a bleach mixture. You might want to have the house inspected and treated if you suspect a more extensive problem.
Make sure to keep ceiling fan blades clean, and turn the fans on at a moderate speed when showing the house so that the buyer can see that they work properly. Always keep the temperature set at a comfortable level depending on the weather.
Remember, most buyers will request a home inspection prior to closing. If there are any problems with the heating and cooling systems, they will probably be discovered during the inspection. Buyers do not want to buy a home with heating and cooling problems so take care to address any problems prior to putting the house on the market. It will save you time and money in the long run because you won't need to delay a closing waiting on repairs.