KANDLA, India—As heaps of castoff clothes from the U.S. cascade down a conveyor belt, rows of sari-clad women frantically sort the garments by type—T-shirts in one barrel, women’s jeans in another. They pluck out sweatpants, underwear, sweaters, coats, and even furs.

The jumble is part of the thousands of tons of used clothing that arrive each month in this western Indian port, a hub in the vast global network that purchases secondhand clothes in rich countries and resells them throughout the developing world.

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