It's not illegal, per se, to sell cold medicine, antifreeze and aluminum foil.
Heck, I buy cold medicine, antifreeze and aluminum foil several times a year (maybe only once or twice a year for antifreeze).
That's not what's against the law.
I gather that what's against the law is selling it to drug dealers -- how on earth can you know what the purchaser intends to do with these items?
Not at all.
The law is written such that the clerk must know that it is being used for illegal purposes. That's why the undercover guys are telling them.
The whole point of the article is that these immigrant clerks "don't understand" English or the slang being used; therefore, they should not be prosecuted for selling 50 boxes (or whatever) of Sudafed to one person.
Too bad. Learn to speaka da English.