We have a "Contradictions in the Constitution" thread here and I put an essay on my blog as a result of that. The contradiction is that it is not clear when Congress has MONOPOLY powers. Only Congress can declare war, for instance, Ohio cannot. Having your own navy and granting yourself letters of marque and reprisal would be problematic. Having your own post office would not.
I believe that in many ways, the government needs the power to carry out its business, for instance, by operating its own television or radio stations just as it has websites, but that these should not necessarily prevent others from being in a business.

As noted Congress has the power to keep and report a Journal. At first, the Senate chose not to, and other private individuals reported on the Senate. No one sought a writ of mandamus forcing the Senate to keep a Journal.
In 1800 and 1802 radical Republicans sought to close the US Mint as drain on the public purse. Again, Congress had the power, but might have elected not to use it, so would NO ONE be allowed to coin money?

So, yes, it is a great convenience for the government to make government coins and other instruments, but that should not prevent other people from creating their own.

No, Van Nostrand did not become Borders, but like many businesses, Borders is gone. That raises another issue with numsmatists. As strong as is their knowledge of money and love for it, you do find governmentalist idiocy now and then. When they released a new book about Wildcat and Obsolete banknotes, Krause bemoaned the fact that these instruments were not as stable as government money today. The irony works on several levels, but just to say, businesses come and go and it is not a sign of immorality or dishonesty, but simply growth, change, and evolution. ... and Borders is extinct, sad to say...