Costco is the largest chain membership-based warehouse shopping mall in North America, committed to providing members with high-quality products at the lowest price. Costco's products cover a very wide range, including all kinds of daily necessities, health products, fresh and fast food, snacks and desserts, drinks and alcohol, electrical appliances and furniture, and even prescriptions etc. But not everything at this warehouse store is a good deal, some items sell for way cheaper at the local supermarket than here. So What are the best things to buy at Costco? What should you not buy at Costco? Here are 18 products that are definitely worth to buy at Costco and 6 others that need to avoid to buy.