What is the test for an individual to be found guilty of a charge of accessory to murder for the sale of a firearm in a criminal case?

Other examples may be cited. In Backun v. United States (1940 4th Cir.) 112 F.2d 635, it was said: "To say that the sale of goods is a normally lawful transaction is beside the point. The seller may not ignore the purpose for which the purchase is made if he is advised of that purpose, or wash his hands of the aid that he has given the perpetrator of a felony by the plea that he has merely made a sale of merchandise. One who sells a gun to another knowing that he is buying it to commit a murder, would hardly escape conviction as an accessory to the murder by showing that he received full price for the gun; and no difference in principle can be drawn between ... a seller who knows that the purchaser intends to use the goods which he is purchasing in the commission of felony." (112 F.2d at p. 637.)