"With respect to trading sugar, if they give it away for free at restaurants, you probably don't want to be trading it" -John L Person 1961
Conversely, when a 5lb. bag of sugar has become a 4lb. bag it is probably worth trading. Domino, pure cane sugar, 5lb bag is 4lb. People are adverse to price increases but will not notice this. Check your sugar.
I personally detest when someone says "Never do this", there are many ways to skin this cat, least we need is a Kool Kat telling us the do's and don'ts. Trade and let trade. Peace.
"The moment everyone wants something, it becomes overvalued" (something along these lines) from Liar's Poker, on the popularity of Investment Banking as a career right after under grad.