Sellers and Purchasers are unlikely to ever encounter this situation but it is handy to know where you stand in the event that you do.
If the Seller passes away after the offer has been accepted the sale agreement remains valid and should be upheld by the executor of the sellers estate.
If the Purchaser passes away, the executor of the estate has the option or choice to proceed with the purchase or not.
In any event the transfer is likely to be delayed somewhat while the executor is appointed and paperwork is set in order.