I understand that the law states that you can not sell an automatic weapon but nothing says you can not own such a weapon.
That means you can buy the parts necessary to make the modification and it only cost a few bucks.
Nothing you said is true at all.
You need a Class III license to buy/sell automatic firearms.
You can’t purchase an automatic trigger group without that same license.
Fully automatic weapons are legal to own in many states in the US. You have to do the proper paperwork and submit it to the BATF along with the fees, and wait for approval (weks to months or longer).
The going price for a legal full auto M-16 is in the tens of thousands of dollars these days.