Even if the government were inclined to act like the Gestapo many people on this thread seem to fear, they don't have the time or the resources to follow up on every cash purchase. (I know many people are going to disagree with that last statement and tell me all the examples they know, have ever heard of, or made up. Please save the bandwidth. I have stated my opinion and it is based on 30 years of personal experience dealing with the government. I believe you are all entitled to your opinion. I ask you allow me mine.)
Not right now. But believe me, if history teaches one thing, it's that a government that is granted an arbitrary and excessive temporary power, will never give back that arbitrary and excessive power, and will use that arbitrary and excessive power in ways you never imagined. It's called: Unintended Consequences.
The accretion of power to the center acts like a black hole, a gravity-suck pulling more and more authority, until you have full-blown totalitarianism. Wiser men than you or I have known this: they wrote the Constitution specifically to shackle the central government and curtail this inexorable phenomenon for as long as possible.
Or maybe they just realized that people are inherently dumb and/or corrupt, I haven't decided which.