There are a handful of rare varieties but many inexperienced and/or deceptive sellers list the common varieties (with billions printed) with the rare varieties (some have <100 known). The differences are subtle (i.e perforation gauge) and requires a bit of learning and understanding to differentiate them from the common stamps.
You can learn about this herehttp://stampsmarter.com/learning/ID...arities.html
For some reason, many folks want to believe that they have stumbled across the rare varieties. This is like finding a lottery ticket on the sidewalk and thinking that it is the winning mega-lottery ticket.