With the DEA's actions in the Medical Marijuana sense, I think that vendors would actually be afraid to sell it. Still, growers would be off the hook, unless this tax bill is some kind of underhanded way to increase the sentencing on marijuana crime. The first law against Marijuana was the Marijuana Tax Act of 1937. Basically it made it legal to possess marijuana if you had the tax stamp, although the government only issued a small amount of these stamps.
Also, if a drug is legal in a State, but still listed as a schedule 1 drug, the fed can harass shop owners and threaten them with legal action. In fact, a DEA agent can wait outside a State permitted marijuana store and arrest every person coming out. The charges may not hold, but they are still in their Federal rights to confiscate and detain. What we need is LESS FEDERAL INVOLVEMENT in how we live our lives.
I know that people will see through the Fed's lies and threats and still sell it, but if it becomes impossible for anyone to get the paperwork to sell it, then well, it's one more small step to victory, but a step none the less. On the flip side, the guy selling it without a license may even be in more shit than he would have been. The "less than an ounce" loophole may become obsolete because it's no longer drug trafficking, but tax evasion.
They really fuck you for that.