Steam Guard will be required for all trading done through Valve's Steam service beginning next month, as shown in a post on Reddit from user reireirei.
"Starting December 12th, Steam Trading will require that your account has had Steam Guard enabled for the last 15 days before you can trade items," reads a screenshot from Steam Trading linked on Reddit.
Introduced in March 2011, Steam Guard is a form of account protection that requires email verification when logging in from an unrecognized computer or browser. Used in conjunction with Steam Trading, it would prevent users from creating new accounts and immediately trading or gifting any rewards they earned with purchases.
Users in the same Reddit post speculate that the policy change may be tied to an upcoming sale. According to the Reddit users and a thread on Steam's forums from last year, during its 2011 Winter sale, Valve included special achievements which, after earned, became "coal" that users could trade for items. By creating multiple accounts and gifting the rewards, some users exploited the system.
The new requirement also comes in the wake of an alleged Russian trading scheme on Steam in which users apparently purchased in-game keys using fake credit cards to launder money.
We've contacted Valve about the changes and will update this article with more information as it becomes available.