Interesting link that somewhat ties to this topic
http://www.heavypen.com/coins/page3.html
The seller can claim anything to be rare so it's up to the buyer to distinguish. "Rare" to me is a Coelacanth fossil. I have to catch myself when considering a large baseball card purchase because they are not that old and I tend to feel better about shelling out money when something is both old and rare. One hundred and fifty years is hardly recognizable on a time scale for someone who collects fossils or say paleolithic artifacts. "Old" and "rare"' are both relative because no card is alike and "old" could refer to earlier issues. Beanie Babies from the 1990's could be considered old. It's not incorrect to claim something as rare regardless of it's population or age but it is subjective because what it is measured against is up to the seller. I've always found that cards are more "rare" when you're trying to sell or trade them.
My personal scale for a "rare" baseball card is 100 or less known examples.