An unprecedented collaboration between a European coin seller and a U.S. certification company is the latest evidence that the international collectible coin market is booming. Numismatica Genevensis SA (NGSA), one of the leading European rare coin auction houses has contracted Numismatic Guaranty Corporation (NGC) to certify approximately 300 of the 1,500 pieces, including many ancient coins, being offered in its December 2-3 auction in Geneva, Switzerland.