I'm open to correction, but I don't believe there is any one law that applies to this.
There are three parts to it, as I understand it:
1. You can't buy from a FFL without filling out the form
2. The FFL can't transfer the firearm to you if you answer "no" to that question "Are you the actual buyer of the firearm"
3. You can't lie on the form.
These three points combine to make it impossible to buy a gun from a FFL on someone else's behalf without someone breaking the law somewhere along the line.
If I am right about this, then:
1. If you take someone else's money and lie on the form, you're guilty of (a) conspiracy to commit a crime and (b) lying to an FFL on a government form - you're guilty of those 2 even before you actually transfer the firearm to someone else!
2. Straw purchases don't apply to private sales since there is no form to lie on
As I said, this is just my take on it from the reading I've done of the 1968 GCA and the ATF website. Don't take this as gospel and certainly don't take it as legal advice.