So, if you're a criminal and you don't wish to suffer Al's fate, what do you do? Well, clearly you declare your ill-gotten income on your tax forms. Problem solved!
Ah, but that's only income tax — what about sales tax?
Well, you buy a revenue stamp. Here's a genuine example of one, which my friend Frank has just sent me from Texas:
I'm now apparently entitled to sell 1g of marihuana to a citizen of Texas without risking a rap for tax evasion.
There are a number of characteristics of this stamp that I like. Firstly, it's made of a reflective material so you can't photocopy it (what, don't they trust drug dealers to be law-abiding?). Secondly, it has the mass of the marihuana stated in grams, rather than the usual US standard of ounces, presumably as a concession to the business practices of the dealers (who seem to prefer metric weights and measures). Thirdly, it leaps at the opportunity to advertise the ill effects of drugs by putting them between death and taxes in its logo — that's certainly going to give those pushers something to think about!
The best thing, though, is the line of thought that caused such things to be created. The government wants its taxes, the dealers don't want to be jailed for evading taxes, so a revenue stamp addresses both their needs. It completely misses the point that in buying a stack of these stamps, the dealers are effectively incriminating themselves: they may greatly reduce their chance of being arrested for tax evasion, but they greatly increase their chance of being arrested for drug dealing.
Needless to say, most of the people who buy these things are collectors rather than social reprobates. This is why Frank is unlikely to have the FBI call in on him to check whether there are pot plants in his plant pots...