For those of you who might encounter some problems when making Korea-related purchases online, do switch to using Internet Explorer if you are not already using them! E.g. air tickets (Korea Air, Asiana Airlines or Jeju Air), Gmarket etc.
This is because, to facilitate ecommerce and enhance online security, a law was passed in the late 1990s that allows purchases only be made through Internet Explorer. IE currently still dominates South Korea.