Yes, it is.
My first thought when I read the headline was, "what was he doing illegally?" but then it talked about him making and selling them illegally so it made more sense.
Among the regulations in the states and towns I've lived in:
-can't have more than 3 cars registered to an individual;
-can't brew more than 20 gallons of beer per year;
-can't have more than 3 dogs on your property.
And there are plenty of residences that prohibit everything from fences and tall flags to RVs. You can't bring more than 2 ounces of liquid on a plane in one container. Sharp objects are banned, torch lighters are banned. Many states still ban oral sex, or gay sex.
So the idea that a person who already has several guns should be questioned, checked, or restricted trying to buy many more guns seems perfectly reasonable. There are plenty of weapons an individual is prohibited from buying; so when does the number of legal guns equal the danger of a prohibited weapon?