I could support that I suppose. Criminalize the selling but not the buying. But I could see exceptions to that. For example, in Pennsylvania, a person can put himself on a self-exclusion list that makes it a criminal offence punishable by 3 months in jail and a heavy fine if he is caught entering a casino. This might deter some problem gamblers. If they didn't think it would work, why else would they choose to put themselves on such a list?
The same principle should apply to the consumption of mind-altering substances other than as prescribed by a physician. In other words, a drug addict who wants to add his name to a national five-year-automatically-renewable self-exclusion list that would make it a criminal offence for him to consume any mind-altering substance other than as prescribed by a physician under threat of a heavy fine for doing so should have the opportunity to do so. It should be voluntary of course since it might not be the solution especially for hard addicts. But for soft addicts, that might just be the kick in the pants that they need? Why deprive them of that? When dealing with such harmful substances, we should not impose such freedom on those who do not want it.
