Pokémon Green and Red were the first releases in Japan. Green version was not released in the United States, making it harder to find. A worldwide release of Pokémon Red and Pokémon Blue in English were mass produced shortly after, making them more common.
Pokémon games were released for the first time in 1996, and after their initial success were widely produced and available. While used copies of the games are common on the secondary market, sealed copies can fetch much higher prices. Unopened copies of the English Red and Blue versions have been sold for over $400 in 2014.