Gun law in the United States is defined by a number of state and federal statutes.  Sale or delivery of firearms is restricted in Missouri like any other U.S. state.  In almost all the states of the US, an importer, manufacturer, dealer or collector shall be licensed under the federal gun control act of 1968 to be a lawful seller.

In the state of Missouri, any political subdivision, or any person shall not prohibit or restrict the lawful possession, transfer, sale, transportation, storage, display, or use of firearms or ammunition during an emergency[i].

As per the laws of Missouri, a person commits the crime of unlawful refusal to transfer by denying sale of a firearm to a no licensee. who is otherwise not prohibited from possessing a firearm under state or federal law, solely on the basis that the non licensee purchased a firearm that was later the subject of a trace request by law enforcement.  However, this shall not apply to any individual federal firearms license holder, his agents, or employees to the extent they chose in their individual judgment to not complete the sale or transfer of a firearm for reasons specific to that transaction, so long as those reasons are not based on the race, gender, religion, creed of the buyer[ii].

[i] § 44.101 R.S.Mo.

[ii] § 571.014 R.S.Mo.