Such a ban won't work anyway - and cash is legal tender - so the fed preempts states on that. What they were probably trying to ban is transactions that don't result in sales tax collection.
Good luck suppressing the gray market!
Any government has to depend on some cooperation - we can't all be police, and an all-police state has its own issues anyway. But governments always seem to try and boil that frog, and put more and more things under their control. The thing is, when it gets to a point where it's perceived they're skimming too much - they lose the cooperation required to make it all work.
The tighter your grasp, the more slips between your fingers.
Edit:
As NRA and VCDL life member, we see states and localities passing laws all the time that are preempted by federal statute. It's a PITA to get them taken off the books, but we generally manage to, and even make progress. It usually takes a court case and some money to get it brought up in the correct venue, though.