Ethics of con artists

[Spongebob]

What about MLMs.  They don't lie to you, they tell you the truth, that you could make a ton of money if you get people on your downline.  You wont, but it's true that you could make a ton of money if you did.  In the meantime, you voluntarily part with your hard earned cash because you believe in your ability to succeed.  

Lying and stealing a bad things, sure, but you don't have to lie or steal to con someone.  You just have to know one thing they don't.  The entire food labeling and marketing industry is based on this.

That's a valid, and far more subtle, point.  At what point is it a con?  If I learn a secret that everyone wants, I can market that, even if the secret is a simple thing that anyone could do.  For example, would it be unethical if I learned of a substance that could make a car go 300 miles per gallon of gas and it turns out the substance is something really simple and easy to produce?  I patent it and put it on the market for $10 an ounce and get rich, but of course it also helps everyone who uses it because one ounce turns a tank of gas into 10 tanks.  I don't think its a con because the buyer gets what they paid for even if it turns out the secret ingredient was just cat piss.  But its certainly a con if the substance does absolutely nothing except maybe damages the engine.  A product similar to this was marketed a while back.