This. It's admittedly pretty simple if you want to go buy a firearm in a reasonable state, like Maine or New Hampshire. You go to the gun dealer/sporting goods store/Walmart. You pick out what you want. They submit your information to the background checking system. You go get lunch and come back an hour later. Assuming you're a law-abiding citizen, you pay the cashier and walk out the store with your new firearm.
Now, what you can also do legally in most states is purchase a firearm from another private individual through a private sale. An enormous number of second-hand gun sales occur in this fashion, through Craigslist or classified ads or personal acquaintances. Effectively, this is no different than if you were buying and selling lawnmowers or comic books, or any other mundane object. I am skeptical of any attempts to really regulate this activity, because it is effectively unenforceable given the number of firearms currently in circulation and the almost complete lack of any records of ownership history for most of them.