Two kinds of object, two different problems, usually on the same shelf.
Books
Light fades spines, and a run on a bright shelf becomes visibly uneven within a few years — which is exactly what a buyer sees first.
Humidity foxes page edges and warps covers.
Pressure from a tightly packed shelf damages spines.
Store them upright, supported, not jammed, out of direct light, off the floor, away from an outside wall. Dust jackets where an edition had them are worth a clear cover.
Figures
UV yellows pale plastic permanently, and fades box art. Humidity softens cardboard, which for a boxed figure is most of the value. Weight crushes windowed boxes.
Store them out of light entirely, upright, nothing stacked on top, with silica in the box during the monsoon.
Displaying
Displaying spends condition for enjoyment, which is a fair trade made consciously. Keep the display out of sun, and when you sell, the word is "displayed" rather than "mint". Describing condition.
What it costs
Shelf position, which is free, and a few boxes. The costly version is a complete run with three faded spines.
Questions people ask
Does spine fading matter to buyers?
Yes, and it is one of the first things noticed on a run.
Can yellowed plastic be whitened?
Treat it as permanent.
Should books be in plastic?
Covers for jackets, yes. Sealed bags long term, no.