Military Camps can be both permanent and temporary, but regardless the function is the same. This is a place where soldiers have setup a camp as a base for training and requesting items.
While there is an active Military Camp on the map they will periodically request different kinds of goods based on what the soliders need and what they are consuming.
If you have something the soliders have requested you can take it straight to their camp and offload it. Military Camps will pay a set price for the item, which means unlike the market, you can always expect a consistent price.