If you saw a house on RealtyTrac, it does not mean it is necessarily on the market. RealtyTrac is a site that tracks pre-foreclosure activity. If you saw a house on there, it merely means the homeowner is behind on payments. It is best to have your Realtor check to see if those houses are listing on the MLS.
Realty Trac is merely an information service. To find out if a property mentioned on Realty Trac is available or not your best bet is to contact a REALTOR in your area who can search MLS and other records to find out the actual status of the property.
Realty Trac is not a listing site,nor are they authorized to list properties. They publish notices of default & foreclosure which they get from the public records. These houses have not yet come on the market & the prices listed are not accurate. If and when these properties do become available they will be listed on the MLS at market value which will be determined at that time. If you are interested in purchasing a foreclosure,I urge you to work with an agent who is familiar with this somewhat complicated process and who will have access to the MLS.