Answer to who is brokering the oil - Turkey.
Once in the world trade, oil is fungible, and oil obtained from questionable sources is traded just the same as that for which there is full provenance from wellhead to refinery.
Heavy arms can be traded back and forth with relative impunity on the black market, and there are enough “dirty” banks to cover the transaction, so who has the incentive to look into the cash flow on oil purchases?
Certainly one of the best candidates. No doubt there are plenty of Saudis who would be happy to oblige as well.