This law does not govern transactions between private parties. It covers cash purchases in the amount of $10,000 from a retailer. As you well know, such transactions are covered by the Commerce Clause of the US Constitution and reasonable limitations on interstate commerce have been upheld by the Courts.
What if I buy from a local store? All I said was that this Act, as applied in some cases, is a violation of the 4th Amendment. Is it in all cases, no. Will it ever effect me, no. Is that a reason to allow the Government to write sloppy laws that do not follow the Constitutional prohibitions? I don't think so.