All trolling aside my theory was that the spike in Earthbound sales was due to the release of Smash Brothers Brawl (March 9, 2008 in NA according to wiki) and the inclusions of Ness in that game.

Brawl was an extremely popular game on an extremely popular console, and Ness could be considered an "unknown" to many people compared to the likes of Mario, Link and Kirby. Players wonder about the "unknown" character and the game he's from and want to buy the game, demand can't keep up and prices spike and continue to stay artificially high even though there was a decent number of copies of the game produced.