Although it does not happen much anymore it was common place back during the big real estate price run up…. buyers wanting to know what the sellers paid for a property.
Six, seven or maybe eight years ago buyers were frequently gauging the “profit” the seller was making in the then out of control market place. [The "profit" motive eventually led some flippers to financial ruin but that is another topic.]
But, in my experience, this is less common in 2013.
One reason is most properties are selling for less than the seller paid if the purchase was made five or six years ago. Or, if the seller purchased the home ten or so years ago, the list price is probably not far from the market today.
The reality is what someone paid is not nearly as important as what a ready, willing able buyer is willing to pay.
There is an ideal selling price and what one paid is likely not a valid component of the determination.