The government has granted the U$.$S$.$ Postal Service the exclusive right to deliver mail.

No $($not many sellers$)$

There are several dozen crust makers that distribute their frozen products to hundreds of pizza shops nationwide. The shop owners source crusts from the cheapest available producer.

No $($not an identical product$)$

A few major airline companies represent a great majority of total air travel. Consumers consider all different flights to be essentially the same, and comparison shop for the lowest price.

Yes, satisfies all assumptions

In a large city, one chain of coffee shops controls a large market share because locals believe its coffee tastes better than that of its competitors.