Even with diminishing marginal utility for the individual, the community demand curve may not be downward sloping. People may conclude that a person's duck in a restaurant is good because others are eating in the restaurant. People may enjoy eating in the restaurant more if other people are there. People may value the restaurant more if it is expensive.

Draw a demand curve for a restaurant under assumptions of information, network effects, status effects. Do you still have a downward sloping demand curve?

Do you necessarily have an equilibrium price and quantity where demand equals marginal cost?