This subject was raised in another thread I started about buying parts from France. The sale of milsurps in the US is a huge business and there are plenty of collectors eager to buy exotic, ancient weapons. Ancient being WWII or earlier, more or less. The way it works is a cache of weapons and other militaria is found or a foreign government just decides to get rid of the contents of an old arsenal. They put the word out on the market and milsurp dealers inspect the warehouse and make an offer.
Over the 25 years I have bought milsurps the inventory has changed dramatically. Back in the 1980s a SKS rifle was virtually unattainable and even the ammunition was collectible. That changed with the fall of Communism. Iron curtain nations sold off literally tons of them. Within a few years prices dropped dramatically and you could buy huge tin cases of ammo very cheaply. Now, 25 years later the supply is long gone and prices have risen considerably. I used to see racks of Finnish Mosin Nagant rifles for $80! They are hundreds today. A whole collector field is established with dozens of variations in the rifles documented. Books are written. Online forums spring up.
When an arsenal becomes available, the buyers do not closely examine the weapons and bid on their relative rarity. That would take a colossal effort. Instead, the foreign government puts everything on pallets and weighs it and the buyers offer so much per pound. They get it dirt cheap.
Then the importer starts sorting their buy and puts out the word about what they have found to collectors and a feeding frenzy ensues. The most sought after items are pistols and rifles. If a weapon is a full auto weapon it must be demilled, i.e. the critical parts are crudely cut into pieces so they don’t work. The rest is sold as a “parts kit”. Here is a parts kit I recently purchased.
Here is an article about a warehouse in the Philippines that is yet unsold as far as I know. Lots of photos.