Wal-Mart is now the first major US retailer to go beyond selling its own products online, by posting around 30m items linked up through Oodle and its listings.
costs nothing for the seller or the buyer. Products for sale include foreclosed homes, concert tickets, pets, cars and jobs.
, which was launched in 2005 in San Francisco by former Excite and Ebay executives, is in direct competition with Craiglist, the free classified ad site that has 30.3m unique users each month, according to ComScore.
Oodle claims to attract around 5m users per month.
Craig Donato, Oodle chief executive, said: "This is good for Wal-Mart because it provides a different experience for consumers, and it benefits Oodle because it expends the reach of our network."
The new service is part of Wal-Mart's plan to increase the revenues made from paying advertisers on its , which is already the second largest online retail site in the US behind Amazon.com.