Hello Dominique, for sure, Local Markets can't be beaten for sustainability...they are absolutely the best places to shop for fresh produce, but unfortunately we can't all afford to do that. When I say can't, I really mean can't. I'm not describing choice. For some people, saving a euro here and there really does make all the difference and it can be a luxury to be able to spend an extra 5 - 15 euro a week on local/organic fruit and veg. Supermarkets are not all baddies. Some sell local fresh food and because they can stock and turn over more, it doesn't always have to mean less money for farmers. The CO-OPs have a fairtrade policy too.
I had an interesting experience recently at our market, when a "local" farmer tried to sell me garlic at 4 times the going rate. I asked why, and she claimed only her garlic was local, and everyone else was selling cheap spanish imports, but other traders then invited me to see their fields and banned her from returning. She was lying.