The key here is "if intended for human consumption." This is why legal highs are frequently sold as "plant food" or "bath salts." Some vendors are more careful than others, in the height of the first "wave" of designer drugs around ~2004, some vendors were as brazen as to offer "sample packs" and the like. And then Web Tryp [1] (my favorite codename ever) happened.

Many designer drugs are close analogs of schedule I substances.

[1] http://en.wikipedia.org/wiki/Operation_Web_Tryp

That's a US-specific law though. The article is mostly about the UK market, and in the UK the law bans specific chemicals, and attempts to address families of chemicals. It invariably falls short and molecule-tweaks can skirt it.

The drugs mentioned in there - Benzo Fury (I hate that name, let's call it 6-APB because that's the molecule on the wrapper) is extremely close to MDA, aMT is a tryptamine which escaped the generic tryptamine regulations and the NBOMes are a twist on the generic 2C-x structure.

Aha interesting, I didn't realise that.