Just a few thoughts about ammunition sales, since without ammunition a firearm is just an expensive baseball bat.
A couple of years ago I built one of the AR15 clones - for several reasons including because I could and I wanted to see how it worked. I purchased almost all the parts online, legally, and bought the receiver from a licensed dealer - where I had a background check. All above board and legal. (the receiver is the critical part that is needed to make it a functional firearm - the other parts on their own are just hunks of metal and plastic)
When I wanted to take it to the range I needed ammunition. Since centerfire ammunition is not exactly inexpensive, I thought I'd try one of the online sites. It was very easy to order quite a large amount of ammunition online, and more surprisingly other than a trivial check (I asserted that I was over 21 and not a felon) there was little more than a check of my credit card.
Something is wrong with this.
Actually several things are wrong with this.
1. I purchased a critical component of a weapon system without any real background check. The sites didn't even ask for a driver's license number or anything that could be used to check that I was legitimate. As long as visa vouched for me, I was fine.
2. I could have purchased as much as my credit limit would let me. While there is a price advantage to purchasing a few hundred rounds at once, there are some real issues with purchasing more (thousands). They range from "what the heck are you doing with that much?" to "how are you safely storing it?" It isn't that hard to safely store a few hundred rounds (a couple afternoons at the range's worth), but a few thousand begin to get problematic. (This is just like with gasoline - a 5 gallon fire marshal approved can in the garage for your lawnmower is fine - a 50 gallon drum problematic and a 500 gallon tank a real hazard). Similarly if I'm running through thousands of rounds a week, I'm either running a shooting range or redistributing the ammunition.
One thing that would be a very good change in the law is to make the online sales check more than my credit limit. There are a couple of ways of doing this - ranging from requesting enough information to verify my legitimacy to requiring some sort of registration/approval to be transmitted before the sale can happen (some of the online dealers are already a bit better about this than others).
It is also important to keep track of how much ammunition is being ordered. Not simply because it could be used or stockpiled by some private militia (after all it only takes 20 rounds to make a massacre), but also because a large amount of rounds or propellant, improperly stored, is a real safety hazard. I would not want to be a fireman when one of these private arsenals is on fire. Similarly if someone is ordering thousands of rounds a week and isn't running a range, then where is it all going?