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You may not know this, but the most popular way to sell on the web is pay to play. Google Shopping requires sellers to pay to be listed for price comparisons. But not all companies are eager to take part. E-commerce giant Amazon doesn't want to pay, so Google Shopping results don't list Amazon.

The classic test of this is to search for an Amazon Kindle e-reader. Google Shopping has the item listed from third-party sellers EBay, Staples, Quill.com, but not Amazon itself, the company that actually makes the Kindle.

If the system worries you, you could always ditch pay to play shopping engines, or you could soon try a solution called Shopping Enhancer. Karl Quist, who built the service, joins Marketplace Tech host David Brancaccio to explain how Shopping Enhancer will work.

Follow David Brancaccio at @DavidBrancaccio