St. John’s University is in serious negotiations to sell its scenic Staten Island campus property to neighboring Wagner College for $30 million, The Post has learned. A source familiar with the talks said Wagner signed a preliminary agreement to buy the 16.5 acre site in Grymes Hill overlooking New York Harbor. “They have an agreement and they’re working toward the closing,” a source familiar with the talks said. The agreement had a 60-day escape clause that has since elapsed, and Wagner is now