Reasons to shop local first
psfk.com
psfk.com
People don't respond well to nebulous, "this strengthens the community"-type stuff. They like good products, good service, low prices, and a nice user/shopping experience. The crucial point is that whatever the good things are about your business, people have to feel them in a single interaction, the very first time — not years down the road. You can't depend on people subordinating their short-term interests to a vague community benefit that a) won't show up for a long time, and b) only shows up if everyone else in the community also makes the same sacrifice.
In the US, supporting independent, community farmers is key in battling the negative effects of the government corn subsidy. There are numerous places you can learn about this subsidy's adverse effects on developing countries--I'd suggest Food, Inc. for one. Buying locally grown food is also an environmental boon, since there's much less fuel burned in transportation.