Large supermarket chains are absolutely a great example of monopsony power. Producers selling to the supermarket often can't choose their own price. The supermarket can decide the price, and if the producer doesn't like it, he gets to sell nothing at all, even if the actual customers do like the product. And supermarkets absolutely use this power. I've heard of supermarkets deciding to discount certain items against the wishes of the producers, but still letting the producers pay for the discount. Because the supermarket controls the distribution channel, they wield power over both the customer and the producer.