Seems like the biggest difference would be that generally, with a loan, you know something about the person. With a private sale, you don't, you just have to reasonably believe that they're not a prohibited person. Even with loans, in some cases, you don't have to know that the person is responsible, you just have to not know that they aren't. For example, in the document you posted earlier is a case, McBrayer v. Dickerson, in which a kid was held (quoting the synopsis) "not liable for negligent entrustment of firearm to friend who accidentally shot third party absent showing that juvenile knew of friend's incompetence or habitual recklessness".