I don't think I'm getting my point across or you're disagreeing with my point but not saying so.
The markets can't afford for that oil to stop flowing and NEITHER can the entity which controls the oil. We need they oil, they need the money. Let ISIS take control, they still need to govern the land that they control. People still need to eat, people still need to earn income. Oil is manna there.
What we're talking about here is the skim, as the mob used to say. There are costs to pump the oil, costs to keep the infrastructure running and maintained, costs to administration, etc then there are the subsidies for food and gasoline, there are the costs of importing gasoline for the domestic market and so on.
There's no guarantee that they'll even care about the domestic economy in Iraq if they take power. The Taliban certainly didn't when it took control of Afghanistan, and neither did the the Khmer Rouge in Cambodia.
Besides which, even if a group like ISIS were to opt to sell the oil, they might jack up the prices beyond reasonable levels in an attempt to send the global economy into a tailspin. That's what OPEC tried to do in the 1970s, and basically what Saddam's MO for ensuring his own position before 1990 happened to be as well.