Valve today announced further changes to account security and item trading.
As you may be aware, Steam added a feature called "Trade Holds" to help with account security if you did not have your account linked to the Mobile Authenticator. These trade holds were for a period of 3 days; Valve has announced that as of March 9th that period of time will be extended to 15 days for those who do not have their account secured by the Steam Guard Mobile Authenticator.
This extends to placing items on the Marketplace, with a hold period for placing items onto the market when you aren't using Steam Guard Mobile Authenticator.
For the full announcement, read http://store.steampowered.com/news/20631
We fully support the changes Valve are making. As an economy-enabled game, account and item security is of utmost importance to us and our players. We recommend that you protect your account via Steam Guard as soon as possible in order for these changes to have minimal impact.