Sales tricks.

Although the obvious concept here is the salesman fooling the customer, this is not the only way these tricks take place:
Packaging may contain misleading information. This need not be dishonest, just enough to make people misunderstand the facts.
Salesmen, who may not even know the facts. Their job is only to sell.
Advertising is commonly used to spread misleading information. This then gets passed around until, eventually, most people accept it as truth.
Enthusiasts may spread incorrect information. If enough people tell you the same story, then it has to be correct, doesn't it? Some of this may originally come from dealers or manufacturers who then make the sales from the stories.
Experts may appear to endorse a product, but are they really an expert and are they just saying what the company are paying them to say?