Alongside stacks of 50-pound bags of Producer’s Pride Hog Grower pig food and sunshine yellow CountyLine Heavy-Duty Super Spear hay handlers, Steve Barbarick, Tractor Supply TSCO -0.33% Co.’s chief merchant, is rummaging through a bin of weighty grade-5 hexagonal head bolts, the kind commonly used to fasten structural steel. Sure, you could buy similar bolts at Lowe’s or Home Depot HD +0.87% and pay for them individually, but at the Tractor Supply in Thompson’s Station, Tenn. they are sold in bulk and cost only $3.69 a pound. And at those other stores you couldn’t pick up a four-pack of Diamond Classic Heeled Horseshoes on the same trip.

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