This is not across the USA. I think something like 13 states have universal background checks. For instance in PA if we were both residents there and you wanted to buy my shotgun I could sell it to you if I didn’t have reason to suspect you were a prohibited person. Private sales are legal in most of the free world USA.
Even in NYS there are certain familial relations where you do not need to have a NICS check dine, they are very limited but they are there. For instance when NYS did the semi auto rifle licensee and you needing to be 21 to possess a rifle after 9/4/2022 I sent my son an email stating I was gifting him all my semi auto rifles to get around this. Now he has documentation that prior to all this he’s legally able to own/possess my rifles. Also they can be shared for hunting purposes and target shooting so we are both covered.