presumably since you "just bought a few years old " smart, it must be a 450 -the older model, so be wary of the difference when shopping for panels. (Haven't been to ebay lately, but I assume there are more offerings for the newer model?) Body panels are indeed the plastic panels of the car, which are removable. Skins are vinyl graphics which are applied over the panels.
In any event, we do have a post about swapping panels- should be readily findable.
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