[sshine]

The "true if enough people operate under the assumption that it is not true" is funny and somewhat intuitive:

If you know markets are efficient, there is no reason to haggle about the price.

But haggling is the mechanism that enhances price efficiency.

[speedplane]

In a truly efficient market, you wouldn't need to haggle about the price. You would have a slew of options that you could choose from, and would choose the option that best fits your demand. In an efficient market, if you were ever being overcharged by a company, another company would pop up overnight, and you would immediately switch from one to the other. No haggling required, just knowledge of what's available and easy movement from one option to another.