So you do the right thing and buy your groceries at the farmer's market, organic produce, yadda yadda. Then one day you run out of toilet paper and go to the evil chain supermarket. And while you are there, you pick up (non-organic, non-local) milk because you are running low on milk at home, and buy some Oreos which come from the evil food conglomerates, because Oreos are fucking tasty.
And yet, this does not diminish you at all. You are living in two systems: the local-food-consumption system and the mainstream-economy system.
Farmer's market (CC-BY) - http://www.flickr.com/photos/nataliemaynor/2539111053/
Supermarket (CC-BY-NC-SA) -
http://www.flickr.com/photos/earthworm/130590895/