After spending years fighting Amazon.com Inc. to force it to collect sales taxes, U.S. states are turning their attention to the individuals and small companies that account for a growing share of the online marketplace’s sales.

Starting April 1, the Seattle-based giant will expand its sales-tax collection to all 45 states that tax sales. But most third-party sellers remain elusive, giving them a pricing advantage—about 10% in some cases—over local brick-and-mortar retailers and products sold directly by Amazon.

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