Most of these comparison sites (or engines) allow merchants to pay for better placement in search results. These merchants usually pay a portion of the sale or a flat "price-per-click" to the comparison site. They can also pay be to be listed as a "featured store". This can lead to some pretty biased search results for consumers.
Other comparison sites simply crawl the internet to search for products, also allowing merchants to upload their product catalog for free. But giving merchants the option to upload their products also makes it easy for them to control your results. For example, some merchants won't list their best deals--if they did, they'd get a ton of traffic from customers who wanted to purchase only that discounted product. If the deal really is a true bargain, those sales will cut into the merchant's bottom line and be less profitable in the long run.
Sound confusing? Just remember this: Comparison sites are businesses and as such, they need to turn a profit. Don't use them as the final word in low prices, but rather, a guide that offers a glimpse of products' average market price. Find reputable stores carrying your product at a competitively low price, but remember that sales tax and shipping will probably increase your total. I recommend that you conduct an internet search of your own (try google or bing), especially for merchandise that isn't widely carried. Then start looking for coupon codes and revenue sharing links to save you even more money.
Here are some of the top price comparison engines:
GoogleProductSearch (formerly Froogle)
Shopzilla (formerly BizRate)
Also see:
