If you have been scammed using a bot impersonation scam recently or entered your Steam login credentials on a fake Steam login page, you should follow this guide to secure your Steam Account.
Start off with changing your Steam Password, checkout this guide to do so.
After you have changed the password we will proceed with disabling a possibly hijacked Steam API Key.
To do so, go to the Steam Web API Key Page and press the Revoke My Steam Web API Key Button as seen in the picture below. If you don't see a Key there, you can safely continue.
Now your account should be secure again and you can safely trade, once again.