Several different federal agencies hold government auctions. The General Services Administration is the granddaddy of them all, because it sells on behalf of other departments. When a federal agency no longer needs something — say, a pickup truck — it reports the truck to GSA, which first offers it to other federal agencies and then to state and local governments or nonprofits. If nobody claims the truck, then the GSA auctions it off to the public, and you get your chance at it.
It is important to have realistic expectations when attending a government car auction. While you can find some good bargains, you are not going to find a brand new BMW for $100.00. Government auctions sell both fleet cars and vehicles that have been impounded by government agencies. The conditions of these vehicles can range from great to not running. Set your expectations and budget realistically.