Not content to be an online sales leader, Amazon.com Inc. is thrusting itself into the physical world.

The company on Wednesday began offering a mobile-payment service and smartphone-compatible credit-card reader aimed principally at food trucks and mom-and-pop shops. The service, dubbed Local Register, enters a crowded arena, competing against similar offerings from eBay Inc.'s PayPal, Square Inc. and Intuit Inc., as well as more traditional payment systems.

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