I saw in another thread that it was military ammunition. Some still in the belt for automatic weapons. I doubt if the military sells this as surplus. My guess is that it is either stolen or bought through some foreign source. All this speculation is made on the condition that the whole thing isn't some scam by law enforcement agencies.
They don't as of about 2 years ago. Prior to that, the company that had the contract to demill the .50 surplus was selling the ready to fire rounds and the components. Then, for some reason the components only were available. You could buy all of the armor piercing incendiary, tracer, or ball rounds (bullets) that you could afford or could carry. I know, I bought a bunch.
Just recently, the govt went to the co that was demilling the stuff and offered them the same amount of income that they were getting by both demilling and selling the components if they would destroy the components after demilling the rounds. That is what recently dried up the mil surplus 50BMG. Some of it is still available, but in limited quantities. Try Widener's in Johnson City, TN if you want some.
Another source that is still avail is from Israeli Military Industries (IMI) Widener's, and several other suppliers have that also.
The point I'm making is that this ammo was and IMO still is plentiful enought to not get anywhere near the "rare" category, although articles like this may hasten the day when it will be.