Perusing some for sale listings of guitars, many of them are Custom Shop, i.e. built to customer specification and not off the wall production models. By definition, these are one-of-a-kind instruments. However, many of these listings state them as "rare" which I find as a descriptor doesn't sound quite right when describing one-of-a-kind items.
Is something that is one-of-a-kind considered rare? When I think of rarity, I usually think of it in tiers or on a spectrum: things that are common, things that are uncommon, things that are rare, and then things that are unique (one-of-a-kind). To me, rare and one-of-a-kind are mutually exclusive.
To use another example, consider the Mona Lisa. Is that rare? To be more declarative, what contributes to an item's rarity: availability, desire? Can an availability of one be considered rare?