It is interesting how many sellers will have this in their listing. I've seen ebay listing where the sellers will add "if you don't pay for insurance I am not responsible if the item doesn't get to you." I guess those sellers haven't actually lost an uninsured item in transit yet.
I think those folks have a misguided understanding of where the sale takes place. The deal isnt complete until the item is in the buyers hands and the policies of the payment processing companies, paypal, and the credit cards, confirm this.