Viki Stevenson stands behind the counter, passing fashion judgment. She's wearing a gauzy black Viktor & Rolf blouse and skinny Diesel jeans as she sorts through a pile of clothes in the shop where she works in Brooklyn, N.Y. After rejecting a high-waisted, sequined pink skirt, she snaps up four G-Star narrow-cut cotton T shirts. "You gotta know your brands," she says, as she tosses the keepers into a metal bin.
Stevenson isn't a buyer at an exclusive boutique. She's a store manager who decides what items to accept for resale at a secondhand-clothing shop called Buffalo Exchange. It's part of...