Take newspaper classifield advertisements, for example. There are people who want to sell golden retriever puppies or used Toyotas, and people who want to buy golden retriever puppies or used Toyotas. Classified ads give people to people information and price and quality so that they can make rational, self-interested decisions about what's available for how much. Because many people are participating, no one can sell for too much or buy for too little, and the market works efficiently. Newspaper classifieds, then, are an excellent example of how ads can help an efficient marketplace. Rational self-interested individuals getting useful information about products so they can sell as dear as possible and buy as cheap as possible.