The Illegal Gun Parts resemble a pistol grip and muzzle device; in regions where "assault weapons" are prohibited, the definition of "assault weapon" often includes a rifle fitted with a pistol grip and/or muzzle device.
As is stated on the Sandgun page, the Sandgun has a graphic that resembles the Illegal Gun Parts put together.
The Illegal Gun Parts are likely only sold at night because of the illegality referenced in their name, as well as their implied shadiness.