It is a common misconception that a property that appears on RealtyTrac is therefore for sale. RealtyTrac simply tracks where in the foreclosure process a home is - not the reason it is there or a homeowner's intentions. Just because a homeowner has received a NOD does not imply that they want to sell or are even in distress.
That being said, there is a much better chance that a home listed as REO may be for sale, but even this is not necessarily the case.
Searching for homes for sale using RealtyTrac is akin to assuming that a home with a dumpster in front indicates the owner is getting ready to move. Some certainly will be, but much of the time you will be spinning your wheels. Oh, and sorry about the lame analogy!
If the property is NOD it is usually not for sale, just in the beginning process. REO's generally would be listed with a realtor and therefore would be on the MLS. The info on Realtytrac is not always 100% so you should defintely consult a realtor to make sure you are getting the right information to make informed decisions.