As long as you are not using a lower receiver [1], it's not actually a rifle, just pieces of metal and plastic. The lower receiver is the actual firearm according to U.S. law. For example, these [2] are the non-rifle parts of an AR-15.
ITAR applies to certain accessories as well (optics, some suppressors, and I believe certain muzzle brakes). That's only an issue for export, though.
(and the "lower receiver" vs. "receiver" part is only for the AR-15 or rifles of similar design; lots of rifles have a single receiver, and for handguns it generally means the frame -- it's in reality a circular definition, with the receiver being the serialized part which is serialized because it's the receiver...)
It's really interesting with firearms like the Sig P250, where the interchangeable piece is on the "fire control group" and the rest of the firearm is totally interchangeable -- this is specifically because German regulations restrict people to owning two handguns, and this lets you have one firearm in multiple calibers, sizes, etc.
(Amazon's policy is even weirder..."anything in the firing path" is prohibited, but a lot of other accessories are commonly sold, and the policy isn't really uniformly enforced at that.)
Plus, in some countries they restrict airsoft and pellet guns (like this one) based on muzzle energy (or sometimes velocity). 3-6 J is a common limit.
[1]: https://en.wikipedia.org/wiki/Lower_receiver
[2]: https://i.imgur.com/TozIZ7l.jpg