they're limited-release items. you're there for the bunch of minutes when they're being sold or you have to buy it from a 'reseller'.
gasmasks are relatively easy; you can have up to 10min after 'opening time' before they sell out. any vendor resellers will obviously charge more than the original creator. if you're buying from other people, you're stuck with waiting for a generous person (man...gasmasks are really getting marked up these days..).