I dont see how the limit would or could be enforced. Who is keeping track of private sales? No one that I know of in most states. Either youre an FFL or you arent. If you arent then there is little scrutiny beyond what your state requires...if you even bother to comply.
It would have to be by entrapment, only. In fact, there was a leak that the ATF was not too keen on this because there would be nearly no way to enforce.