This doesn't prevent, but in fact encourages, the black market. Either pay the gov $1000 or the guy in the van $5 (whatever market price is). You already see this with cigarettes in the US, where the price difference between taxed and illegally sold (untaxed) cigarettes are much, much smaller.
Additionally, you'd likely create a black market for whatever piece of paper says the LSD is legal.
I never said I wanted to prevent the black market. I merely stated a scheme under which I would be comfortable acquiring such substances, and the trade-offs likely to be involved (high prices). As for the piece of paper, it was just a figure of speech; passing a law amounts to the same effect.
Additionally, you'd likely create a black market for whatever piece of paper says the LSD is legal.