What is fair market value and how do we determine it?

Fair market value is what a reasonable third party would be willing to pay. The resale value is the same as fair market value. We would expect the resale value to be less than what the jurisdiction originally paid for the supplies unless there is an extreme shortage and high demand, but you should not assume that the value would be reduced without doing some documented market research. 

You can access online reseller resources to determine fair market value and document your methodology.  You could also provide guidance to your counties to access online resources, e.g.  Amazon, eBay or other reseller marketplaces, to see what the unused supplies they have are selling for on the open market.