By Matt Goad A door slams shut behind a satisfied customer and another door opens with a new customer not far behind. This happens all day long, three days a week in the small, cinder-block shack that houses Tom Robinson’s Carolina Seafood of Carrboro. With a small staff, the business manages to get to the North Carolina coast every Wednesday to get fish and shellfish to sell in the Piedmont on the weekend. It brings in salmon from Nova Scotia, and some oysters from Virginia, depending on the season, but almost everything comes from North Carolina. Manager Salvador Bonilla generally travels to Morehead City, Swansboro and Beaufort to deal with the local anglers there. Tom Robinson started the business in Chapel Hill in 1975, selling from the back of a truck parked at Pantana Bob’s bar on Rosemary Street. Bonilla has managed the business for the last 16 years. He didn’t know much about the fish business at the time. He worked as a chef at Vespa, the defunct Italian restaurant on Franklin Street, and …