Qualité observable

In economics, plums and lemons are terms used to describe the quality of a product. Consider a market for used cars. 

Plums are good-quality cars, and lemons are inferior-quality cars, each assumed to make up half the market.

It is assumed that the quality of cars is observable. This means that buyers and sellers can tell if a car is a plum or a lemon. 

It is assumed that buyers are willing to pay $20,000 and sellers value plums at $16,000. 

These cars sell between $16,000 and $20,000. Buyers benefit from acquiring the car for less than their valuation of $20,000. Sellers benefit from selling the car for more than their valuation of $16,000. 

Buyers are willing to pay less for lemons due to their expected limited future use without major repairs. Therefore, it is assumed that buyers are willing to pay $8,000 while sellers value lemons at $6,000. These cars sell between $6,000 and $8,000. Buyers benefit by paying less than their valuation, while sellers gain by selling above theirs. 

Overall, buyers and sellers benefit due to the observable quality of the used cars.