Most likely, you have an API-key generated. Please, check if that’s true using this link: https://steamcommunity.com/dev/apikey
There should be no API-key on any Steam account by default. In case you haven’t created the key yourself, you should delete it. After doing that, try to repeat the trade. Make sure to install our Antiscam extension to protect your account from scammers: https://bit.ly/AntiscamFAQ
Probably, you have Steam Guard restrictions, so you should try to offer the trade to someone in Steam to find out the issue.
In case the trade has been automatically declined again, please contact our technical support.