The appraisal form has four columns, Subject and three comparables. That is four properties to compare not two.
CMA - same thing, at least three comparables.
If you can't find two more then you have to go upsize, downsize, out of area, or out of time and make adjustments. But you have to find two more. Then you can go back to best comparable and state why its the best comparable and why you are using the data from it to determine the value, and not relying so much on the other two.
Obviously the first factor to consider is their locations. 2nd would be size/number of bedrooms and 3rd would be the condition and amenities. In my market, I am finding that currently it is imperative to focus on sales only as recent as the last 60-90 days instead of the traditional 6 months. Anything beyond 90 days in my market is old information, as we are seeing such a rapid decline.
Melissa Mancini, Realtor, CBR, GRI
If you are looking at what needs to be compared without the use of an appraiser, just like the other useful posts so far, everything needs to be considered. The two properties have to be comparable first of all--it's close to impossible to get a reliable comparison if you're comparing two completely different properties in completely different neighborhoods. But if it is comparable, then consider: location, size, bedroom #, bathroom #, upgrades, livable square footage, lot square footage, interior material used, building material used, any necessary repairs, etc. It's a long list that's no doubt! But the more information you have and the more information you can compare, the more accurate your value determination will be. Good luck!