Sellersville gets a new manager next month, when veteran Alan Frick retires.  He was with the borough for 40 years. His successor is David Rivet, who spent most of the last two decades as Public Works Commissioner for Cattaraugus County, in Western New York State.  Borough Council members say they like Rivet’s public works background and his way with people.  The day before his job interview he drove to Sellersville with his wife, visited a park and talked with people at local restaurants.