I think if you are going to disclaim responsibility the place to do so is in the auction description, not in an email sent after the fact.

As a buyer if I am buying from someone I know to be reputable like Mike Richter I would always turn the insurance down. In order to insure a $55 purchase the cost of insurance is $2.00. I would have to suffer a loss rate of more than 3.6% in order for the insurance to worthwhile.

The only time I would voluntarily buy the postal insurance when buying from a reputable seller is when the cost of the item is so great that I could not stand to suffer the loss regardless of how unlikely the loss was to occur.