Tax assesments alone are an inexact way of determining your properties value, but I don't agree that they are of no use. Trends can be extracted from the data by cross referencing recent sale prices and using a more general approach with a lot of data. If you can access 10 or more properties similar in style, age, neighborhood and living area and cross reference against a recently sold properties, you can begin to recognize a trend in the ratio of sale price to assesment value. Keep in mind that you may want to throw out the high and low prices just because they may have information about the condition or features that may not calculate into the assesments. Also, make sure your assesments were done in the same year, as most assesors change there formulas and values frequently. This is a viable tool to use, but your best advice will always come from an experienced and succesful Real Estate Professional.