GameStop depends on used-game products for a quarter of revenue, while margins for used games are about double those of new software. The threat from Wal-Mart helped fuel a 11 percent drop in GameStop shares this year through yesterday, and short sellers are betting on even more pain. GameStop dropped 1.2 percent at 10:18 a.m. in New York for a fourth day of declines, its longest losing streak in more than three months.