Picture a Grandma Moses landscape northwest of Northampton, dotted with barns and sheds. Then picture those buildings stuffed to the rafters — literally — with high-quality handmade pottery, jewelry, furniture, clothing, and glassware, all on sale. Snow Farm in Williamsburg, home of the New England Crafts Program, continues its annual Seconds Sale this weekend and next. Made by 150 crafts artists all over the country, the goodies range from $5 hand-printed note cards to $500 art-glass plates. Visitors can lunch in the campus cafe and observe glass artists at work. It’s time to let this secret out of the barn.