It is clear to me as it should be clear to anyone with an open mind that Turkey is buying ISIS oil. That is not even in question. The Turkish-Syrian border is as porous as a sieve, even though according to president Obama, they have made progress reducing the amount of people and oil coming through there. His exact quote was that there are some gaps. The question now is if the government is complicit in this leaving of gaps.
They recently asked an armed forces guy here in america about Turkey and oil. I think it was the head honcho in the army in Iraq and his reply was immediate. Of course turkey does not help ISIS terrorism, it is a major partner in fighting them. You could physically see him squirming. This is due to the amount of pressure that Turkey has on USA in that America has so many resources and so much money tied into the country that the USA cannot dare critisize the Erdogan government. Even though they harbor suspisions of his actual trading with ISIS in oil. Biden had to actually apologize last year for even impliying that Turkey has traded with Daesh.
With the recent Russian accusations about President Erdogan's family connection with the energy industry and ISIS. There seems to be a clear pattern of co-ordination between Turkey and ISIS. You have to remember that Turkey has had a huge ongoing problem with PKK. What essentially seems to be happening is that ISIS is helping Turkey with PKK, sapping away its fighters and actually fighting aganist them. In return, Erdogan is allowing trading with them to occur. In the process, him and his family are turning up a tidy profit in all this as big players on the energy market in that area. ISIS is killing turks, it is damaging PKK, Erdogan was losing aganist PKK, even had to try bargaining with them last year but not any more, not thanks to Daesh
The attacks from Turkey aganist Russia are make the appearance of the existence of a patnership seem damning,Russia says they were flying against ISIS, implying ISIS was close to or even in Turkey. Turkey officially fired the missiles, the turkmen(Grey wolves who are alligned with Erdogan and help him fight the PKK) kill the pilot, They gave the body back to the government (implying the clear ties btw them and Erdogan) now Erdogan gives back to Russia a dead corpse for burial and wants them to chill out so they can go back to business as usual.
If Turkey was a secular state this would not be happening. Even though they profess secularity,Turkey in the hands of Erdogan is a muslim state, a muslim state that has ties with fundamentalism and ISIS.It is not a bastion of stability in the middle east