The market, with a single vendor and the consumer is not able to buy goods and services from other vendors were as a substitute. Make a monopoly power in the pricing of their products and services.
Market with a single seller and a consumer can purchase goods and services from other suppliers to replace it. Monopolist makes higher power in pricing their products and services.
The market with only one seller and consumers can not buy goods and services from other sellers, so replacement. The monopolist is high power in determining the price and their services.