Rarity is the numerator in the ratio of supply and demand. Every year, people uncover fresh relics and the items find their way to market. Sometimes only one is found; sometimes there are hundreds. On the flip side many collectibles are lost each year because of calamity or neglect.
The collectors themselves are another aspect of rarity. As the population of collectors increases, the rarity of an item increases. For example: If a pool of 5000 Pre-Pro Kessler serving trays exists, are they rare? If 100,000 people want to own one the answer is yes, but if only 1000 collectors want one on their wall the answer is no.