Liz Carlson forked over $70 for a “heavyweight cotton” hoodie after seeing an enticing ad on Facebook that claimed it had “the softest interior we’ve ever created.”

“I love hoodies, so when I saw that ad, it spoke right to my heart,” said Ms. Carlson, a 32-year-old in Oakland, Calif.

The ad was from an e-commerce site called Affinity Find,...