I thought this was rather good from Julian Baggini in The Guardian: although it does have one terrible error in it:

You might dismiss this kind of ethical consumerism as mere gesture. Waving my right-on debit card as a badge of honour as I pack my Fairtrade chocolate into a canvas tote bag can look like a poor surrogate for revolution. This is part of a knowingly superior narrative of impotence that tells us our day-to-day choices can't lead to meaningful political change. "The system" is inherently corrupt and to believe we can affect it by our choices is to buy into the very myth of consumer power that late capitalism promotes in its own interest. It is to believe that virtue can be bought, when the vice of the system is precisely that it puts a price on everything, including a clear conscience.

But this narrative is wrong. It portrays capitalism as though it were a kind of entity with a will of its own, whose only desire is to maximise profit. In fact capitalism is amoral, not immoral. It doesn't care for right or wrong, only for what people demand. If we demand goods and services at the lowest price, capitalism will provide them, and damn the social and environmental consequences. If, however, we demand Fairtrade bananas or recycled toilet paper, capitalism will provide them too, as it demonstrably has done.

These are not things done by capitalism: they are things done by markets. Where there is choice then it is indeed possible for people to have a choice on how, with whom and upon what they spend their money. A capitalism without markets (ie, monopoly capitalism) would provide none of those choices: just as any other economic system without the choice offered by markets would not allow the consumer to express their preferences. Capitalism and markets are simply not the same thing at all and it is markets here that Baggini is praising.

Other than of course he's precisely spot on. If you want things to be produced in a certain manner then it's up to you to spend your money so as to encourage producers to do their production in that manner. You not only can but you ought to express your moral choices in the way you decide upon who to buy from. I tend to buy from factories located in poverty stricken hell holes as that's the best way to alleviate poverty we've yet found. Agreed, others might differ on this: but it's still true that you should deploy your financial firepower to make the world a better place by your own lights.

It is, after all, vastly better to light a candle than to curse the darkness and every pound spent on your moral goals brings them that one pound closer.