The issue is most private gun owners don't do background checks when they sell guns, therefore when a citizen sez "I sold my guns, I don't have them" the gun controllers are going to have a hard time proving you wrong. Remember its illegal to sell to a known person prohibited, its not illegal to swap guns with a casual acquiantance you don't know the name of at a gun shop or firing range. This isn't just a way around the law, this is what real Fudd's and gun owning normies actually do, making tracing real guns difficult to impossible. There is nothing barring you from selling a gun to a random asshole that you yourself bought a week before. Strange, but not illegal, and not unheard of in completely legitimate circles.
Ammunition? Forget about it, it can establish you are an active gun owner who burns through a lot of ammunition, and imply you have a gun. Its useful for that, I'll grant you are correct on that, if you buy 1,000 rounds of anything every month it would make sense you own a weapon chambered for that and use it. If they come "where is you stockpile of ammunition" saying "I shot it all" isn't even a clever dodge, its a legit answer. You can use it to cover up for a stockpile because its an answer they can't refuse, they can't prove you didn't blow all those rounds. Because maybe you don't stockpile and practice.
To complicate that matter even worse, you can claim that you don't shoot or stockpile or even own a gun, you got into ammunition speculation and trading after Sandy Hook and you still do it. Claim you go to a few gun shows and haggle and trade with folks, they would have to do a lot of investigation to prove you wrong.
"Where is your stockpile of Hot Pockets you fat fuck?"
"I ate them all because I am a fat fuck."
Now, how do you deal with such a situation? Beyond all of these factors, how many people buy ammunition online? Can they do detailed investigations of every one? Do you think that your "huge' 500 rounds a month you fire and reorder is so big you are red flagged when there are people who buy twice, thrice, more than that?
From a security point of view, geo political thinkers like myself have made grand criticisms of modern data mining because most of the data is fucking worthless and just clogs up the system and uses resources and area that gets in the way of useful information. "We know what brand socks this terrorist wears". OK. Tell me when that's going to be relevant? "This dood once bought 500 rounds of 5.56 on the internetz 2 years ago". OK, what can we do with that? What can anyone do with some of this wholesale information, and how millions of gun owners buy and trade ammunition? Too much data and not enough use.