Whether you are looking to buy a power tool or neon jeans online, chances are you start your search on Google or Amazon.com. But which one?

Behind the scenes, the two companies are waging a war to become the pre-eminent online mall. And e-commerce sites large and small are caught in the cross-fire. As for consumers, the question is whether they will see a full range of products available online.

Google is a search engine, not a store, but it is increasingly inching into e-commerce with products like its comparison-shopping service, Google Shopping. At the same time, more people are using Amazon , a retailer, as a search engine to look for what they want to buy.

Trying to stave off the competition from Amazon, Google has recently changed Google Shopping to require e-commerce companies to pay to be included in shopping results, so product listings are now ads. Inclusion used to be free.

Google says the change will improve its shopping results because retailers are more likely to list accurate and up-to-date items if they are paying. It says the service had become polluted with product listings that were out of date or misled consumers about things like shipping prices.

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