"Surplus" is just what the name imples.
Some military somewhere in the world changes their guns, and the ammo along with it.
Suddenly, they have a dozen warehouses full of .303, or 30-06, or 8x57 Mauser, or 6.5x55 Swedish Mauser, or whatever that they no longer need.
So onto the civilian market it goes. As it is already paid for by the taxpayer of whichever country it came from, any price they can get for it is good. And as there is a literal boatload of it showing up all at once on our shores, the price is held low.
But eventually, it runs out, all bought up.
The original country is no longer making it, why should they? They dumped it because they changed, remember?
Now the price goes WAY up, due to (limited) supply and (big) demand.
Any guesses what will happen to 7.62x39 once all the cheap Czech stuff is bought up?
That what i thought too. I just bought 2 more crates of 1200 today from Lever Arms