Newly Authorized Device
If you are logging onto Steam from a device that has not been Steam Guarded for 7 days you will not be able to trade or use the Community Market from this device for 7 days. Any other devices that have already been authorized for 7 days will still be able to trade and use the Community Market.
Actions such as clearing your web browser cookies, using a new web browser, reinstalling Steam, and reformatting your computer will trigger this restriction.
If you have had a Steam Guard Mobile Authenticator on your account for at least 7 days, there will be no trading or Market restrictions for using new devices/computers.