You are probably referring to the "listings" posted on Trulia through a certain company.
Most of the homes this company posts on Trulia, are NOT on the market and may never be. This company posts iinformation from public records, whereby the owners of these homes have just been served a notice of default on a loan of some type, or back taxes etc. However, if you read the "listings" this company posts, there is a caveat to the tune of ..."the homeowner has been served a notice of default. The owner can still pay off the loans, ....", meaning the owner may not lose the home and it is not in foreclosure just yet.
Then they expect you to sign up for foreclosure listings.
If you are seriously considering a foreclosure, your best bet it to check the website of the county sheriff in the county you are looking in, where the TRUE foreclosures and their auction dates are listed.
Bear in mind, with a true foreclosure, in most counties you will be required to have a certified check for 20% when you win the bid, and typically you will need to balance of the purchase price withing 30 days of winning the bid. These homes are often not able to be viewed, and are most always sold in as is condition (no repairs made).