AMSTERDAM (Reuters) - Sales at Apple's (NASDAQ:AAPL - News) online music shop iTunes Music Store fell off a cliff in the first six months of 2006, according to a recent survey. Since January 2006 the number of monthly iTunes transactions has declined 58 percent, while the average size per purchase declined by 17 percent, leading to a 65-percent overall drop in monthly iTunes revenue, U.S. market research group Forrester said in a survey among North American consumers. "It is too soon to tell if this decline was seasonal or if buyers were reaching their saturation level for digital music," Forrester...