Generation 4

While I didn't own any of the Generation 4 games, they were the earliest games I could remember. As such I do have a soft spot for them, along with many others. They were the first games to utilize the dual screens of the new DS, would change how physical and special attacks worked, and was the first iteration of the Global Trading System - the GTS - which allowed you to trade with anyone from anywhere in the world (with a wi-fi connection). Generation 4 consists of Pokémon Diamond, Pokémon Pearl, Pokémon Platinum, Pokémon HeartGold, and Pokémon SoulSilver. Diamond, Pearl, and Platinum focus on the new Sinnoh Region, whereas HeartGold and SoulSilver are remakes of Generation 2's Gold and Silver games. Diamond and Pearl are the cheapest of the five, with Platinum, HeartGold, and SoulSilver easily clocking over $150 dollars in some cases. Those three games are considered some of the best games period, which might explain the price.

Games

Pokémon Diamond (x2 USA, x1 PAL), Pokémon Pearl (x2 USA), Pokémon Platinum (x2 USA, x1 JPN), Pokémon HeartGold (x2 USA), Pokémon SoulSilver (x1 USA, x1 PAL)

Shiny Pokémon

Diamond, Pearl, and Platinum introduced one of the most interesting, strangest, but yet the most productive Method Hunts; The Poké Radar. By using the radar to "chain" 40 Pokémon in a row, your odds for a Shiny are boosted to 1/200 chance. The specifics are more intricate than that, but once you get a hang of it, this method hunt can snag you Shiny Pokémon by the handful.

Milotic Sprite Milotic (x1), Psyduck Sprite Psyduck (x1), Golduck Sprite Golduck (x1), Graveler Sprite Graveler (x1), Bidoof Sprite Bidoof (x1), Shinx Sprite Shinx (x1), Ninjask Sprite Ninjask (x1), Shedinja Sprite Shedinja (x1),
Buneary Sprite Buneary (x2), Girafarig Sprite Girafarig (x3), Eevee Sprite Eevee (x4), Slakoth Sprite Slakoth (x1), Nidoran Female Sprite Nidoran Female (x2), Starly Sprite Starly (x2), Larvitar Sprite Larvitar (x2),
Gyarados Sprite Gyarados (x2)