Google is the undisputed king of search in all but one lucrative and vital category: Product searches. Over the past decade, Amazon has transformed itself from a seller of books online to the place Americans turn to when they want to search and buy just about anything — from diapers to flat-screen TVs. In some cities, Amazon has started delivering fresh groceries. With each product search that starts on Amazon instead of Google, the search giant’s main business of selling ads alongside search results weakens. Though Google over the years had experimented with letting consumers buy goods with the help of services such as Google Wallet and Google Checkout, it accelerated this strategy in 2013 with Shopping Express. The service lets shoppers buy things from local retail stores through Google, which then delivers them to consumers from the physical retail store on the same or next day.