In the USA, civilian and personal possession of machine guns that were manufacturered and civilian owned before (some specific date) in 1986 is legal. In 1986, they closed any additional guns going into civilian ownership. Now, we just pass around the ones that have been in that state for the last 22 years.
So, if you know someone who owns a machine gun legally in the US, and you want to buy it, you can, providing your state doesn't prohibit ownership (about 35 states allow it, and not CA, duh). The process of transfer requires federal paperwork, local law enforcement signoff, and a $200 fee (in the case of machine guns), which is a tax paid on the transfer. The process used to take a very long time, but now it's down to just a few months.
Glock 18s were originally manufacturered in 1986. It's possible that there are just a few that missed the cut-off date and this guy has bought them up.
A class 3 dealer, who pays a SOT (special operating tax) can deal in machine guns. However, that dealer can only purchase and stock pre-1986 machine guns or something called dealer samples, which were produced only for a short period of time *in* 1986. In order to purchase a gun for inventory post 1986, the dealer must have on file a letter requesting a demonstration of that firearm from a law enforcement, military, or government agency. They're typically referred to as 'love letters' in dealer slang.
Now, law enforcement isn't so keen on simply writing these up. Every Glock 18 owner I know is a Class III dealer with a love letter and owns post 1986 guns.
Purchasing a dealer license to further your personal collection is a felony, but nevertheless, many hobbyists who want to own modern machine guns go into business to allow them to 'stock' (own) them and 'demo' (go out and play) with them. It's a fine line. To do it, plan on running a legitimate business, and enjoy the perks of modern full-auto firearm ownership. Have a lot of friends in law enforcement or government who actually will write you demo letters for the fun stuff.
Otherwise, fall in love with older full-autos, but get your wallet ready. Since the population was frozen in 1986, and the number can only decrease if they are destroyed or somehow illegally handled, the demand continues to surge, and prices are very, very high.
The process for purchasing a machine gun is the same for all NFA weapons, which inculde short barrelled rifles, short barreled shotguns, any other weapons (like pen or cane guns), destructive devices (bores over .500 or rotating cylinder shotguns), suppressors (silencers), and machine guns. Machine guns were the only ones cut off from ownership in 1986, newly manufactured guns from the other categories can be purchased with the $200 registration tax and federal paperwork (except AOW, which are $5). You'll be fingerprinted and write that check for every transfer. You'll need to notify the ATF in writing if you plan to take it over state lines. And you'll be waiting months for every step of the process.
Confusing? You bet. Easy to misstep? Entirely. Most serious NFA weapon collectors have become lawyers as hobbyists to protect themselves. And, don't forget the liability if one is stolen from your property and used in a crime -- most have vaults for their collections. Not safes. Vaults.
Check your state laws, then set aside some time to learn the federal laws. Then find a reputable dealer, and secure your residence. Have many thousands of disposable income. Find a place where you are actually legally allowed to fire it too. Then, enjoy responsibly.
Feel free to PM me with any specific questions and I will do my best to answer.
For values - check the classified ads at sturmgewehr.com or subguns.com