Well, there has been trade between Iraq and Syria going back even to when Saddam Hussein ruled, especially, in his later years. It was illicit trade that violated UN sanctions. Then after that there was some trade between Bashar al-Assad in Syria and the new Iraq. Also, Syria was a transit point for supplies going into Iraq during its rebuilding.
Since the beginning of the uprising, a lot of those more formal and regularized networks have been broken and now there is simply a lot of cross-border trade between Iraq and Syria, because the border between Iraq and Syria no longer exists. That also complicates any kind of effort to strangle or to isolate or to de-incentivize or incentivize either the Syrian government or the Iraqi government when it comes to areas that are controlled by ISIL.