On the contrary, this is an example of good capitalism. It's all about supply and demand; at $40, our demand will be lowered. By complaining and not purchasing, we increase the supply while decreasing the demand, which will lead to a lowering of price.
The difference between this and liberalism: We don't demand that the government force those mags to be sold at a lower price. We may think the shopkeep is greedy, but recognize their right to be so.