Most of the houses are listed for sale, but if the owner does not want it to be public information he can ask his Realtor not to list on MLS. The Realtor can than try to sell by networking with possible buyers or with other Realtors and other possible marketing ways allowed as per their agreement.
All the home listings on http://www.har.com/ are listed for sale with Realtors. HAR stands for the Houston Association of Realtors and is their website.