The apparent links between ISIL antiquities sales and the raising or revenue are touched on in a recent Reuters article: Ulf Laessing Islamic State used Mosul museum as tax department Reuters Wed Apr 5, 2017.
After they seized Mosul two years ago and destroyed the priceless Mesopotamian artifacts in its museum, Islamic State militants found a practical use for the building - they turned it into a tax office. [...] In a basement room under the main exhibition halls, there was a pile of envelopes used to issue orders to pay Islamic tax, one of main sources of funding for the militants. "The Islamic State ... seeks to levy your duties which were forced by God on the rich people's money," read a message on the envelope stamped with the group's black-and-white flag. The "Diwan Zakat", or Islamic tax department, then left a space for names and file numbers to identify the payments. Next to the tax receipts were green leaflets with Koran quotes, from the same department based in "Nineveh Province", whose capital is Mosul. [...] The militants searched the building methodically for valuables, even breaking up the ground floor in their search for vaults with artifacts inside that they could sell, according to Federal Police officers. Apart from taxes, oil sales, antiquities smuggling and ransom from kidnappings were also sources of income for Islamic State.