Six years ago, Amazon quietly began placing product ads on its sites in the hope of boosting sales -- but now the retailer is embracing advertising more seriously, even for brands that don't sell their products through Amazon. The retailer has sold ads to big brands such as Lexus and Chrysler but says it isn't reliant on ad revenue and won't run ads that aren't right for its user base. "We don't spend time on the competitive landscape. ... The way we think about it is, 'What's right for our customers?' " says Amazon sales chief Lisa Utzschneider.