In 2016, the Bureau of Alcohol, Tobacco, Firearms, and Explosives issued a guidance expanding its ability to regulate gun sales. Historically, a federal firearms license has not been required by the many Americans who occasionally buy or sell firearms at gun shows or private sales. This guidance expands the criteria subjecting gun owners to federal licensing and opens up the possibility that a private person who only sells one or two guns could be forced to register as a gun seller. If you sell a gun without realizing the guidance requires you to register, you could be imprisoned for up to five years and assessed up to $250,000 in fines. Although the Guidance was never submitted to Congress for review under the Congressional Review Act, ATF has quite recently confirmed that it intends to enforce it. However, under the CRA, the guidance is not in effect, should not be enforced, and remains eligible for congressional disapproval.
Guidance here: https://www.atf.gov/file/100871/download
Time article describing the Guidance as intended to “close gun show loophole:” http://time.com/4166989/gun-control-obama-executive-action/
Letter from ATF to counsel for National Association of Arms Shows, confirming that Guidance remains in effect: https://www.atf.gov/firearms/docs/guide/advisory-opinion-licensing-certain-gun-show-sellers/download