Craig Beauregard and Julia Edwards were at Costco shopping for groceries in December when they spotted a deal that was too good to pass up: a one-ounce bar of gold. Beauregard, 33 years old, checked his phone and saw the bar’s retail price of $2,069 was $3 below its on-the-spot price in financial markets. Costco, which started offering gold bars last year online and in a few stores, has been selling out within hours.