Why not just sell them on eBay?

PM Says Iraq Lost 2,300 Humvee Armored Vehicles in Mosul

"In the collapse of Mosul, we lost a lot of weapons," Abadi said in an interview with Iraqiya state TV. "We lost 2,300 Humvees in Mosul alone." 

While the exact price of the vehicles varies depending on how they are armored and equipped, it is clearly a hugely expensive loss that has boosted IS' capabilities.