It seems that gamers will need to have Steam Guard active in order to keep trading on their Steam account for at least a few days.
According to a post on Reddit, some community members have noticed that Steam will require users to have had Steam Guard activated at least 15 days before you can trade items. The picture (seen below) shows that starting 12 December, gamers will need at least 15 days of active Steam Guard before they can trade items.
It’s not too limiting but if you enjoy trading items in Team Fortress 2, DotA 2 or Portal 2, it's suggested that you activate Steam Guard on your account so you don’t have to wait 15 days before you're allowed to trade again.
Never fear though because Steam Guard is actually something really great which adds security to your Steam account. It will just warn you when someone else tries to log into your Steam account from a different computer and will require a special access code sent to your Steam contact email address.