As far as rarity goes, it's a tough call unless it's a limited edition.
Disney may have ordered hundreds of a pin then didn't expect to sell many of, or millions of a pin they thought everyone would want.
And if a pin is no longer sold by Disney, there's no way to tell if all the pins are sitting in drawers somewhere or if they'll show up on the pin board or the auctions.
If you have specific questions about specific pins, you might want to ask on a board like this, since the people who are actively trading will probably have the best idea if they've seen it around.
