They don't have a rarity because they are not boss drops.
Each ID is only sold once.
That means everyone has an equal chance of getting a specific ID.
The only exceptions are IDs that are not sold regularly, or at all.
This only includes 0-99 (Not sold, some used), 666 (Not sold), 1337 (Not sold), and 10000(Auctioned)
Rare as in, there are less "low" ID's than "high" ID's.
If you're going by that, then there aren't that many more high IDs.
Lower IDs usually means that the user is more experienced, I'm just saying that high IDs aren't bad.
No. It just means that key was bought before higher ones.