This. Guns contain many wear items.

So what the ATF does is take an essential part not substantially influenced by wear and declare it to be the gun. Trade in anything that sufficiently resembles this part is treated as trade in guns. Other parts are not considered guns, they're just pieces of metal or plastic. Then there are the parts that you're not supposed to have. But is that an oil filter or a silencer? When it's on the gun it's obvious, when it's listed on a website as an oil filter...