Can you buy and sell auto knives?

I live in texas where it is illegal to carry switchblades but legal to own one if its a collectors item. My friend has quite afew and is in the military. He claims that he is not allowed to sell them because they are military items but it seems like they are bought and sold through private sales in states that allow knives for carry. So what if he went to a gun show in Arkansas where switchblades are legal and sold them some in a private sale and used his original purchase receipts as a proof of purchase in case the army ever tried to track down those knives. I believe private sales are quite common actually. Is this legal, opinions of military and law enforcement are greatly appreciated.