A few times each month, Anne Zarraonandia drives from her Mill Valley, Calif., office to a nearby retailer such as Walgreens or Toys ’R’ Us to scout for bargains. Armed with a price-checking app on her iPhone, she trawls the aisles, filling her cart with toys, electronics, sporting goods and packaged food.

Ms. Zarraonandia, 58 years old, isn’t looking to save a few bucks for her family. Rather, she’s looking for items she can resell on Amazon.com Inc.—at a profit.

...