Easier said than done. Even if the cars originate from within the United States, they pass through so many brokers and middle men that it will be quite a challenge to track their final destination. I would even argue that ISIS doesn't source their cars from the US. Instead they look to the United Arab Emirates, who remain the number one importer of used US car (in containers) by quite a margin. [1]

[1] http://www.wcshipping.com/hubfs/D_OfferPDFs/PDF/2015_Q3_Fina...

I think the challenge is that markets in Africa are quite fluid. The car's don't get sold directly to ISIS - they get sold to a broker, who then sells them to another dealer, and so on..