In a category where the packaging is often most of the value, storage is mostly about the cardboard.
What actually destroys them
Light. Plastic yellows under UV and box art fades. Both are permanent.
Humidity, which makes cardboard soft, wavy and mouldy, and which is the main reason a boxed piece loses most of its value on a shelf.
Heat, which softens adhesives and can deform thin plastic.
Weight, which crushes boxes from the top of a stack.
What to do instead
- Out of light entirely for anything you care about — opaque storage, not a glass display in a sunny room
- Upright, not stacked, so no box carries another's weight
- Inside wall, off the floor, silica gel in the storage box
- Never a loft, which is the worst room in any Indian house for this
Displayed pieces
Displaying is a choice to spend condition for enjoyment, which is a perfectly reasonable trade. Just know that it is a trade, keep the display out of direct sun, and say "displayed" rather than "mint" when you sell. Describing toy condition.
Loose pieces and accessories
Bagged by set, labelled. Most of the value lost in this category long term is a small accessory that went missing in a box of other things.
What it costs
Opaque boxes and silica. The real cost is floor space, and the real saving is not having to explain why a boxed piece looks like that.
Questions people ask
Does yellowing reverse?
No. Keep it out of the light in the first place.
Is a glass cabinet fine?
Away from sun, yes. In a sunny room it is a slow oven with a view.
What about original plastic film on a box?
Leave it. Removing it is permanent.
More: what they are worth, common mistakes.