This is what the acaeum uses:
5: The super-rare (and/or unique) items. Rarely will these items come up for auction, if at all.
4: Very rare.
3: Uncommon, borderline-"rare".
2: Less common.
1: Routinely available in auctions. Includes most
RPG items published after 1980, with a few exceptions.
(from
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Though i'm not sure how many surviving copies those numbers equate to...