Google Inc. (GOOG) has unintentionally provided a sneak peek at what appears to be a looming expansion into classified advertising — a free service that might antagonize some of the Internet search engine's biggest customers, including online auctioneer eBay Inc. (EBAY)

Screen shots of the experimental service, dubbed "Google Base," (search) appeared on several Web sites Tuesday shortly after the legions of people who dissect the online search engine leader's every move discovered a link to a page inviting people to list things like a used car for sale, a party planning service and current events.

Google confirmed the development of the service a few hours after taking down the link.

"We are testing new ways for content owners to easily send their content to Google," the Mountain View, Calif.-based company said in a statement. "We're continually exploring new opportunities to expand our offerings, but we don't have anything to announce at this time."

By offering a forum that would enable people to sell goods and services without paying for the advertising, Google might hurt eBay (search) — a major buyer of the online ads that account for most of Google's profits.

EBay depends on the fees that it receives for helping to sell all kinds of products and services, including items that might be listed for free on Google Base. The San Jose, Calif.-based company also owns a 25 percent stake in Craigslist (search), a popular site that offers free classified ads in more than 100 cities.

Google also has confirmed it's working on an online payment service, but CEO Eric Schmidt has said the service won't compete with eBay-owned PayPal.

Another free online classified ad service also would pose another financial threat to newspapers, which already have been squeezed in the cities where Craigslist provides free listings.

If a free Google listing service materializes, it could change the way many Web sites view the online search engine leader.

Through most of its seven-year existence, Google has depicted itself as a vehicle for delivering people to other destinations that contained a desired piece of information or product.

But during the past 18 months, Google has increasingly been adding more content and services that are turning its Web site into more of portal — a sort of one-stop shop for information and commerce.

"As soon as you start competing with some of the people that you are indexing, it creates a completely different dynamic," said Craig Donato, chief executive of Oodle.com, a search engine that pools listings from dozens of classified advertising sites.

"Google can get away with a lot of stuff, but [Google Base] would certainly give people pause," he said.

Google's diversification has coincided with tougher competition from Yahoo Inc. (YHOO) and Microsoft Corp.'s MSN — two longtime portals that have been trying to build better search engines.