Amid a devastating period for most retailers, videogame seller GameStop Corp. has posted big sales gains, in part by aggressively marketing used products to cash-strapped consumers.

GameStop this month reported a 22% jump in overall sales and a 10% increase in sales at stores open more than 12 months for the holiday sales period ended Jan. 3. It didn't hurt that videogame sales climbed 9% in December to a record $5.3 billion, according to market research firm NPD Group.

GameStop's...