For almost two years, Edgar Guzman has been getting calls from irate customers who responded to his ads on Instagram and Facebook META -2.35%decrease; red down pointing triangle. Or at least they thought they were his ads. Guzman’s company, called “Half-Off Wholesale,” offers home-improvement supplies and garden equipment in bulk out of a suburban Atlanta warehouse. The ads offer steep discounts that are mouthwatering to bargain hunters: $29 for a pallet of power tools, and...