Even though I wrote the previous entry on supporting local businesses, here we can explore the complicated nature of shopping. A decision on where to purchase and what to purchase can be difficult if you weigh all the options, but especially social and environmental responsibility.
One of my favorite sources for grading products and companies is Better World Shopper. Even with a product like gasoline, there is a range of grades.

Better World Shopper ranks on a company level but there are other aspects you could weigh.

Let’s take music buying as another example. What’s better, driving to Ventura to support local business Salzers or downloading the music from iTunes? From an environmental aspect, the download is superior. However, some research has indicated if you are already at a location to buy music then purchasing the actual CD is better.
Check out the Better World Shopper site. They also have an iPhone app