The business world is full of conventional wisdom that gets passed from office to office. Sometimes that so-called wisdom is, well ... unwise. (And occasionally it's really stupid.)

Here are the five dumbest beliefs that people have about "how to sell":

1. The Customer Is Always Right

This myth has been repeated so many times that many people think it's a law of nature. In fact, though, customers are frequently unreasonable and overly demanding. A big part of selling is educating such customers so that they have more realistic expectations. This means telling the customer he's wrong when he actually is.

2. Customers Know What They Want

In fact, customers frequently often have bizarre ideas about what they want and need–and, consequently, about what they ought to buy. Don't cater to these whims. It's up to you, as a responsible seller, to figure out what’s actually needed and provide your best opinion about how to satisfy that need.

3. Every Prospect Is a Potential Sale

If you think that everyone is a customer, you'll end up pursuing fictional opportunities. (This is called "chasing garbage trucks, not Brinks trucks.") If you're selling something, your No. 1 job is to eliminate prospects that don’t have enough money to buy your offering or don't have enough need to justify the purchase. That will let you focus on your actual prospective customers.

4. You Should Never Take 'No' for an Answer

When prospects have all sorts of objections to buying, you're probably wasting your time trying to sell to them. Sales opportunities are like buses; another one comes along every 15 minutes. Don’t obsess on any one deal–and always remember that if you hear "no" more than once, it means "no."

5. The Best Salespeople Are Extroverts

Many (even most) of today's sales situations are best suited for people who are a little bit introverted, and better at listening than talking. In fact, some of the best and most effective sales training programs available today are based on listening techniques originally developed for psychologists and counselors–who aren't known for being extroverted.

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