Assessed value versus selling price is an interesting number to look at, but buy itself is not a reliable guide to base an offer on. Tax assessments are based on general information, potentially unreliable square footage of the home and do not consider substantial upgrades or severely defferred maintenance into effect.
If you are planning to sell or buy a home you have a couple of choices to determine how to best determine value. Hire an appraiser or an experienced knowledgeable local agent.
As an agent I use many different tools to consider where to price a home, one of which may include comparable sales assessed price versus sale price, but also the most similar general sales, dollars per square foot and competitive homes currently on the market.
One other factor to weigh into your question, 2009 assessed values are considerably higher than the are for 2010. Make sure if you pursue this that you use the same year for all your comparisons. Best of luck in your real estate looking.