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.