On the "rareness" of items, I ALWAYS simply go on the number of past examples of that exact item we have ever auctioned in the past.
This is slightly complicated by the fact that after so many years (25), we now regularly get the exact same examples back a second time. And of course, sometimes they have been restored since we first had them.
But there is an added rareness factor. There are some titles where NO movie paper of any size shows up, and others where only a single poster of a different size has shown up, and others where only a lobby card or two has shown up, and others where only a still or two has shown up. I see these factors as adding to the "rareness" of items.
I feel such qualities are worth pointing out. If you disagree, ignore it. I DON'T refer to how many are "known to exist" because there are likely thousands of collections I have never seen, so how could I know that? And I don't look at other online databases when discussing rarity, because many contain the same item that was "auctioned" again and again and I don't want to try to determine which are the same.
There are specific posters where only one has been previously auctioned by us, others where only two or three have been previously auctioned by us, and so forth. I include this information when it seems worth pointing on. I usually DON'T include it on items like silent lobby cards, because those are generally so rare that in almost all cases they have never been previously auctioned by us, so why point it out every time.
An interesting case is the collection of 23 silk banners that were just discovered and consigned to us. That is double the amount we auctioned in all our previous 25 years of auctioning, and I thought that fact well worth mentioning on each auction, because the chances of any of these coming to auction again is extremely tiny.