I think a voting system for rarity which does not publish without the approval by those verified to own a full set or have some experience with a given grouping.
Voting by itself would see a lot of artificial inflation by those earthbound type games, valuable but otherwise common. Not to mention those trying to drive the rarity up artificially. An authority should be used, but all the users should have input one way or another.
Also this could happen on a yearly or even every two to three year basis. The goal is to catch titles like 3 ninjas kick back on snes and avenging spirit in gameboy. Games which were common sealed at a given time period, but when those stocks washed up, the games became phenomenally rare.. The goal would be to treat the rarity guide like a census, happens every so often, documents the way things are, but for the most part, doesn’t bring many surprises.