There is a distinct difference between companies refusing to take forms of legal tender (checks, for example), and a state government outlawing the use of fiat cash.
Very rarely would I say that the Feds have the right. In this case, the US currency certainly should trump state law.
Some jurisdictions have stopped lemonade stands for operating without appropriate licensing.....
Yeah, that's why I can see this law actually being enforced in thrift stores and the like, where it doesn't even accomplish its ostensible goal. It's just that, on a Saturday morning, there's a lot of yard sales going on. And unless you start requiring people to keep books, how do you prove anything? A little bit of cash means nothing. If you have no checks, well, you haven't sold anything yet. I could think of more creative ways to do things, like staking out the place or undercover stings, but that's a lot to go through for, uh, nothing.