Costco’s Gilded Age may be coming to an end. The Big Box retailer is restricting purchases of its iconic gold bars after savvy buyers clamored to cash in on the precious metal amid market turmoil.

The price tag for a 1-ounce South African-made gold bar? $3,279.99, much higher than the $2,000 they were going for 19 months ago, shortly after Costco began selling them. However, that price ...