The matter of you buying an auto online depends entirely on which state you live in AND which state the seller lives in. There are a lot of misconceptions about this, many perpetuated by ignorant companies and, let's face it, law breakers, so my apologies if this comes out sounding a little on the aggravated side. The irritation is not directed at anyone on this forum.
By Federal law (
US code Title 15, Chapter 29) it is illegal to sell an auto over state lines unless it is in the pursuant to a
contract with the US military. It is very common myth that this law has an exception for law enforcement. It does
not. It also is mistakenly believed by many that just being a member of the Armed Forces in and of itself allows you to legally buy an auto under this law. This is as accurate as servicemen being able to buy a live AT4 anti-tank weapon when off-duty because they think it will look nice over the fireplace. § 1244 of the chapter is pretty clear in who it exempts.
Beyond not being able to sell or ship over state lines, federal law contains no other provisions that effect those of us living in the 50 states (as opposed to Guam or other territories). If you live in the same state as the seller, federal law does not apply. However, you do still have to obey the laws of your state. Some states forbid even mere ownership of autos. Some have no restrictions and you can carry them in your pocket. It all depend where you live.
There are plenty of sites that will sell to anyone with money. And they get away with it too. Doesn't change the fact that they are law breakers. Mail order companies get raided and/or fined all the time for doing this. And some just steal your money and never mail anything. Caveat emptor.