Owning the weapons are legal from that date, but the selling of them is not except under specific circumstances. Since the parent said "selling", you should probably learn reading comprehension before spewing your own mouth off in the future.
The requirements for obtaining selective fire weapons in the US is age 21, FBI background check (which you have to pay for), sign-off of local police, and they're also pretty damn expensive all by themselves. School shooters like this deranged kid or even his mother are not going to get their hands on automatic weapons like this, and none of them have.
You do not know what you are talking about. I suggest you kindly not talk about subjects not in your area of expertise, just as I do not post BS on subjects not in my area of expertise. First off, I own Title 2 firearms. Yes, you read that right. I own machine guns, suppressors, and a few other regulated, legally registered weapons. I have been a collector/investor for many years, so I have a clue. The parent post you replied to was correct as well.
Title II firearms are regulated by the National Firearms Act of 1934, commonly referred to as NFA. Sometime the weapons are incorrectly refered to as "Class3" by the general public. All NFA weapons are tracked with mandated registration with the BATFE. Weapons regulated by the NFA are Title 2 weapons (Title 1 weapons are "normal" firearms you see in most gun stores) include Destructive Devices, Suppressors (aka Silencers), machineguns made prior to May 1986, Short Barreled Rifles (SBR), Short Barreled Shotguns (SBS), and lastly the Any Other Weapon (AOW) category. These items can be purchased and transferred to you from any FFL who has paid an annual Special Occupation Tax (SOT) on file with the BATFE. Not all states allow you to purchase or posses these items, but most do. That being said, Federal law is clear that you can own these items if you pass the background check done by BATFE, which generally takes a few months due to the volume of applications they process. State law however, can limit or prohibit your possession, as well as use of the items. When you purchase any of the items above, you pay a one time $200 transfer tax to the BATFE per serial numbered item. The only exception are the AOW's which have a $5 transfer tax.
The age restrictions on the Federal level for Title 2 weapons is the same as it is for a handgun. The age limit to purchase one of these items through a dealer, or to have it transfered through a dealer, is 21 years of age. If a person is between the ages of 18-21, you can purchase a Title 2 firearm from an individual, as allowed by Federal law. A dealer is not involved in a person to person transfer, only as long as both individuals reside in the same state. Background checks are still done by BATFE, and there is a paper trail. Again, these are legally registered weapons.
Any individual can sell a registered transferable Title 2 weapon at any time they wish, to a qualified person, entity, or agency, which passes the background check. The background check is free, however you will pay the transfer tax of $200 ($5 if it is an AOW). The paperwork is filled out and sent in with the appropriate transfer tax amount. When the approved transfer papers are returned to the owner of the weapon, then the physical transfer of the weapon to the new owner may take place. No sooner.
As for machineguns made after May 19, 1986, individuals cannot purchase or posses them. Only dealers, manufacturers, and government agencies may purchase or posses them. These guns are commonly refered to as Post Samples, for they are made after the ban in 1986. Dealers who sell Title 2 weapons commonly have them, and sell them to local LE agencies.
If you want to own a machine gun, be prepared to pay a handsome price. You can buy a car, or a house, for as much as some of them can cost. It is estimated that there are just barely over 100k transferable machine guns in the US that an individual can legally own. They are quite rare, and priced accordingly. See here for an example of prices:
You can learn more about the National Firearms Act here:
On a personal note, it is sad that some people do stupid things, and take lives for no reason. My heartfelt sympathies go out to all involved in the tradgedy. We as a nation should be realistic though, laws do not stop criminals who are determined to do harm. If one tool is not available to him, he will find another. For example, on the same day of the shooting in Connecticut, a mentally deranged man stabbed 22 children in a Chinese school. http://www.bbc.co.uk/news/world-asia-china-20723910
There needs to be a better solution, rather than restricting the constitutional rights of law abiding Americans. As to what the solution should be, that is up for debate. I would like to see a police officer in each school myself. Heck, there was one in the high school I attended many years ago, in a rural community.