What is the difference between a property assessment value and selling price?

I saw a property that was assessed for taxes this year (2010) at 140k and yet, the selling price was 300k. I have never seen this huge a difference in the two values.

Am I missing something? I thought that the assessed value of a property and its current selling price would be close to the same price especially if it was a current year assessed value. When there is such a huge gap and the property is vacant, is there some leeway in negotiating a lower price?

Hope someone can shed light on this.