Perhaps What Troubled Me Most....
About this is, the original poster said he had other stuff of his friend that he was storing, and didn't have a problem storing it, and wouldn't even think of selling. But in the case of his friends gun, since the OP wants to buy a different gun for his own use, he actually asks the opinion of those here if that's cool?
Here's my answer: If you ain't got a problem keeping your friends stuff OTHER than the gun, but you think you have a right to sell off his gun to get funds for a purchase you want to make, I am very happy you are not my friend.
You sound much like a relative of mine, who actually feels if he borrows something from you, and keeps it, that's cool, because he has possession of it right then, and you don't.
Exactly what part of FRIEND don't you get anyway?