You fell for bargain traps. By Magazine Editors April 11, 2011 TIPS 27-32 The Costco effect I love shopping at Costco. I've gotten everything there, from socks, shirts and shampoo to a 46-inch LCD TV. But when it comes to food, I may love Costco a bit too much. And it sometimes requires ingenuity to head off waste. Last September, for the smoked-fish-and-bagel platter I would serve at my post-Yom Kippur breakfast, I bought a 5-pound bag of red onions for about $6. The bag contained about ten onions, but it turned out I needed only two. Let's just say that this may have been the first break-fast in history to have guests walk away with onions as party favors. -- Manuel SchiffresBuying when you can borrow Be honest: How many times have you bought something you've used only once or twice? In an age of social networking, collaborative consumption is the next big thing. At SnapGoods.com, you invite contacts to join your network of borrowers, then post an ad to share or borrow just about anything, from a circular saw to a photo scanner to a Vespa scooter. It's a great way to audition a product before buying, if you must. (The site and its networks are currently spreading nationwide.) You can also swap children's clothes and toys at www.thredUP.com, and trade books, movies, music and games at Swap.com. Buying too much car One big mistake new-car shoppers make is basing their choice on sticker price alone. Factor in the ownership costs of the vehicles you're considering. Service costs (including standard maintenance and repairs), insurance, fuel and depreciation over the life of an automobile add up to big bucks. For example, the base-model Chevrolet Malibu ($22,735) costs the same as the Honda Accord, but its five-year ownership cost ($36,766) is nearly $1,500 more -- mainly because it has a lower resale value (that is, more depreciation). To compare ownership costs of vehicles on your shopping list, go to our guide for five-year service costs, one-year insurance costs, annual fuel costs and three- and five-year resale values. Advertisement New-to-the-lot used cars You may think you're getting a deal, but if that used car has been on the lot less than a month, the price is likely to fall further. A study by CarGurus.com shows that the majority of used cars listed for more than 30 days have had at least one price drop. Ask the dealership how long it has had the car: The longer a vehicle has sat on the lot, the more receptive the dealer will be to lowering the price. Impulse purchases Attractive store displays are designed to provoke impulse purchases, for which you're likely to overpay. Curb your urge to spend and instead head for the Internet to get a deal. Our favorite sites: DealNews.com, which has a team of deal hunters keeping their eyes on a million products at more than 2,000 online retailers; DODTracker.com, for tracking deal-of-the-day sites; and for product price comparisons, we like Amazon.com, Pronto.com and Google Product Search. By the time you get home to your computer, the temptation to buy may have passed. Prepaid hotel rooms When you book a hotel room through a travel site such as Expedia.com or Hotels.com, you're required to pay in advance. Instead, use the sites to compare prices, then call hotels directly to see whether you can get the same rate -- or a better one -- without paying in advance. You can usually cancel without a one-night-rate penalty if you call one or two days in advance. See more ways to stop wasting money.