With the new unannounced update it is now necessary to enter a captcha to trade items on steam. Valve is trying to strike harder against trading bots which are coming from outside of their market. In the previous update Valve already implemented a feature which forces you to keep a gift more than 30 days in your inventory before you'll be able to trade it with other users. You can read about this update here
Valve is trying to strike against other markets. For example sites like CS:GO-Lounge won't work until they find a way to solve the captcha's. Valve said that this update was made to save users from scammers and some reliable Trading-Services will be excluded from the Captcha control. What do you think about the Update, let me know what's on your mind!