Mmmm .... if it's banned by international treaty then you're probably talking about things like chemical weapons. It costs a lot to build furnaces capable of destroying mustard and VX. We had tons of that stuff in storage; we've been trying to get rid of it for decades and it has cost a lot to burn what we could. Ammo for atomic artillery is also banned by treaty and pretty darn expensive to get rid of. Other things like artillery/tank rounds and mortars for WW2 and Korean War-era munitions just aren't going to be sold to the public.