We Libertarians want to decriminalize drugs so that the psychoactive substances you mention will be of CONTROLLED origin, the same way that tobacco and liquor products already are. Who do you sue if you buy a bad batch of drugs from a dealer? Answer: no one. Same question when they are regulated by the marketplace. Answer: the company (or pharmacy) that sold the drugs to you. Decriminalizing drugs will mean that they are REGULATED, because the marketplace is the most natural, organic and fairest kind of regulation there is. It provides (in most cases) almost immediate remedy for defective products or inferior services when government gets out of the way. Fear of competition not government bureaucrats is what keeps companies from giving us inferior products and services.
Sounds nice, if it weren't for the insatiable and destructive appetites of so many drug afficionados. Why did some heroine addicts on methadone maintenance (back at its inception, before they corrected it) use to hold the liquid methadone in their mouths until they got outside the clinic, and then—having spit it into cups— sell it on the street for...money...for what?
For heroine. You can't get properly stoned on methadone maintenance, which is a clue to why your neat market formulae do not apply to psychoactive substances. You are thinking much too rationally for the world of drugs and pleasure.
You do want to legalize all substances don't you? Maybe I'm a bit dense, but does anyone else have difficulty imagining, in this litigious society, that a pharmacist might be tempted to say NO to selling a suspiciously jittery person ANOTHER 8-ball of pharmaceutical cocaine? Or that said person will bat an eyelash before running to the nearest still-thriving dealer he knows who'll be glad to?