Coffin sellers are double agents because vampires are repeat customers who rely on word of mouth. They feed disinformation to the hunters, setting them up for failure or ambush. The double agents are sometimes being used to settle scores amongst the vampires, of course. They get just enough real kills to keep them on the hook.

There’s no reason for a vampire not to make coffins on their own. They have plenty of time to pick up new skills and it’s more feasible to have someone in the community to make it without the involvement of food.
Also it’s difficult to build up a loyal customer base for a product that lasts for decades or centuries when you yourself will only be active for a couple decades.

My username has never been more relevant.

Anyways my business hours are 8pm to 3pm during the winter months and 10pm to 5pm during sommer months.