It seems that Valve will ask all players who are interested in using the Steam Trading system to enable the Steam Guard starting with December 12, in order to make sure that no unauthorized gamers use the service.
Users on Reddit
have exposed Steam’s plans and it seems that all those who want to trade will have to run Steam Guard for at least 15 days in order to prove they have seen no breaches.
Apparently, the move is linked with a number of incidents after the 2011 Christmas sale, during which users set up multiple accounts in order to get lumps of coal and then access free games.
Steam Guard is designed to make sure that all attempts to access an account are legitimate and Steam has been encouraging players to turn the feature on since it was introduced.