Steve Tadelis, Distinguished Economist at eBay and his colleagues have done an experiment and seem to have concluded that its a waste of money to pay for sponsored links on Google when Google’s regular search algorithm shows links to eBay for free.

Before you read the rest of this post, go to Google and try searching for “Amazon.” You’ll probably notice that the top two listings are both for Amazon’s website, with the first appearing on a light beige background. If you click on the first — a paid search ad — Amazon will pay Google for attracting your business. If you click on the second, Amazon gets your business but Google gets nothing. Try “Macys,” “Walgreens,” and “Sports Authority” — you’ll see the same thing.

If you search for eBay, though, you’ll find only a single listing — an unpaid one. Odds are, after marketers at Amazon, Walgreens and elsewhere catch wind of a preliminary study released on Friday, their search listings will start to look a lot more like eBay’s. The study — by eBay Research Labs economists Thomas Blake, Chris Nosko, and Steve Tadelis — analyzed eBay sales after shutting down purchases of search ads on Google and elsewhere, while maintaining a control set of regions where search ads continued unchanged. Their findings suggest that many paid ads generate virtually no increase in sales, and even for ones that do, the sales benefits are far eclipsed by the cost of the ads themselves.

This is a dilemma for Google.  Because it suggests that the way to capitalize on the popularity of their search algorithm is to discriminate against those who don’t pay for ads.  But unless Google reinvents itself as a full-blown paid advertising site, such discrimination is likely to raise legal issues.