They won't ship to MA because the MA AG has sent them a letter and threatened them with legal action. Legal action would require them to appear in court, and pay a lawyer to argue their case. Even if they win, they have to pay for the lawyer--so it is easier for them to just avoid the whole situation by not shipping to MA.
Kudos to Target Sports (and others) for deciding to keep shipping to MA in the face of the MA AG's threats.
To answer your question directly--there is no law (yet) against ordering metal parts, and no law against having those parts in a box in your basement, and no reporting requirement for such parts, be they new or used. That could change, I suppose, depending on how much MORE control the MA AG decides to try to exert over the law-abiding gun owners here in MA. It would be great if she would maybe try to focus on criminals rather than us conservative taxpaying white guys with spotless criminal histories--but hey, protecting the public from violent criminals (whether documented or undocumented) wouldn't earn her points as a "progressive", now would it? To earn points as a "progressive" she needs to focus her energy on the real enemy--which is us.