Each time a monster(s) is sent to your opponent's GY: Inflict 300 damage to your opponent.
Rulings
Individual Card FAQs
"Coffin Seller" applies its effect when monster(s) are sent from the hand, Deck, or the field to the Graveyard.
If multiple monsters are sent to the Graveyard at the same time (due to "Dark Hole", etc.) then inflict 300 damage only because it is considered as 1 time.
Multiple copies of "Coffin Seller" are cumulative.
Trap|
Continuous