You thought you took home a haul at the farmers market last week, but you’ve got nothin’ on Wal-Mart. The big-box retailer has become the nation’s largest buyer of local produce, planning to purchase and sell $400 million worth of locally grown fruits and veggies this year. Wal-Mart says it works with “hundreds” of individual farmers, and has 50 percent more partnerships with local growers than it did in 2006. During the summer months, says the company, one-fifth of available produce in Wal-Mart stores is sourced locally. An emphasis on local produce — which Wal-Mart defines as grown and sold within the state — keeps the company’s delivery trucks from burning about 100,000 gallons of diesel, slices its customers’ farm-to-plate distance by 672,000 miles, and saves it $1.4 million each year.