NEW YORK (MarketWatch) — Amazon Inc. shares are leading gains in the S&P 500 index Wednesday morning after the company unveiled a new Square-rivaling mobile payments service and scored above eBay in a study of U.K. and U.S. retailers. ChannelAdvisor Corp. , which provides cloud-based ecommerce solutions, surveyed 200 online retailers averaging up to 3 billion pounds ($5 billion) in annual …
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