He Paid $13 for $13,000 Cartier Earrings, and Then the Jeweler Noticed When Rogelio Villarreal bought rose-gold earrings for a price that the luxury retailer said was a mistake, he looked to a Mexican consumer protection law. He later said the company delivered the earrings. Rogelio Villarreal knew nothing about the French jeweler Cartier, he said, when an ad popped up on his Instagram feed last December. He clicked on it, perusing the pages of bling and other luxury items, including handbags, watches and necklaces, each listed for thousands of dollars. Then Mr. Villareal, who lives in Mexico, noticed a pair of earrings: slender studded 18-carat rose-gold cuffs lined with diamonds, priced at just 237 Mexican pesos, or about $13. "I was amazed to see how much the necklaces cost and so on and I said: 'Someday,' until I saw the earrings," Mr. Villarreal, 27, wrote on social media . "I swear I broke out in a cold sweat." He bought two pairs. Later, the price fo...