Costco is the largest wine retailer in the U.S.Wine makes up nearly half of the big box store's yearly alcohol sales. The most recent number Costco shared was $1.69 billion in 2016 wine sales, which placed it as the country's largest wine seller. Costco's wine markups are half that of most stores. If you see this tiny star (*) in the upper right hand corner of a sign, it means Costco has the item on clearance. You know how Costco keeps empty cardboard boxes near checkout registers that are free for the taking?