He Paid $13 for $13,000 Cartier Earrings, and Then the Jeweler Noticed

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…