The way you store your action figure is an
important determinant on their safety.
You never know if your action figures can appreciate in value in the
future, so it’s better to be safe than sorry. There are a tonne of things than
can cause damage to your action figures, the key is to learn how to take care
of them and prevent them from most of these factors.
This may be hard to believe, but the box can cause some degree of damage to some of your collection. Some of them are made of materials that can produce an oily film, causing damage. Materials that are applied to the PVC during production are the main cause of such damages. Plasticizer is applied on the figures to make them flexible and soft; and they also reduce brittleness and breakage. Letting the action figures out of the box will allow the plasticizer to evaporate preventing any damage. If you are worried about damaging the figures when they are out of the box, you can still keep them there as long as you take them out once in a while.
Dust and dirt are one issue when dealing with action figures, but they are not that much serious. However, when you live in a humid area, the combination of dust and moisture can really damage the figures. Be careful when you are cleaning off this dirt, if you are too aggressive you will take off some of the paint and decrease the value of your collections. Another option is to use a dehumidifier around the figures to keep them dry and clean.

Long extreme temperatures will actually ruin your action figures. It is very important to keep them in a consistent cool environment. Extreme temperatures will cause many problems with your action figures. When the temperature is too cold for too long, they will great brittle and break easily. Short periods of cold weather won’t have any effect. Too much heat over a long period of time has an effect on the plasticizer. It will cause the surface to produce an oily layer and develop a shiny look as opposed to the original matte finish. In some circumstances it will cause the figures to melt. Extreme temperature variations have a breaking effect on the action figure. The only solution is keeping them in a temperature controlled environment.
Believe it or not, the sun can lead to damage by fading or discoloration to the action figures. The UV light produced by the sun is the actual cause of this problem. You can avoid this kind of damage by storing them under LED lights. Remember to check the lights out before you purchase them because some of them may actually have some UV lights.
When you are storing the action figure, don’t forget to take the batteries out. Batteries have a strong acid that will damage the wiring of the action figures, making them useless. Keep them aired, away from the sun, out of any heat source and in a consistent temperature if you want to maintain their value.