Right, but if someone "gifts" their buddy a $1000 handgun, and the cops find out a few weeks later, how are the cops supposed prove if cash went in the opposite direction?

I hope this isn't offensive, but that is a relatively "programmer" like way to respond. You rarely have 100%, airtight evidence of crimes. That is why the standard of proof is guilt beyond a reasonable doubt as determined by a jury.

I have my doubts that in the situation that you described that there would be interest and motivation to prosecute, but if that buddy had gone on to commit a felony, or this was a repeated offense, then they would use warrants, subpoenas, and other standard prosecutorial tools to try to find the cache, get electronic records, and compel witnesses to testify. As someone else noted, all that's required is falsifying the the purchase details (Abramski vs US).