... you invest in your local farm, in exchange for 5% off produce. Farmers get capital, and a guaranteed market.

There is already a small movement among the alternative ag people doing exactly this.  The idea is to give the farmer a larger percentage of the consumer price while the consumer gets a 10-15% break.  And it works if the farm is close enough to a large enough population so the numbers all work out.  

The sad fact seems to be the vast majority of consumers are perfectly happy eating rubbery vegetables and fatty meats.

She believed in nothing; only her skepticism kept her from being an atheist. -- Jean-Paul Sartre