Another way for the coffin to work. Since it's a shop for the soon to be deceased, you go there to buy a coffin for a specific person. You don't get a coffin, but the coffinmaker goes to that person and measures him/her/it for a coffin, a not so subtle hint someone is looking to kill that person. Then he returns and says the coffin will be ready when the body is.

Ex.

buy coffin bob

The coffin maker takes your coins and grabbing a small bag walks from the store.

Bob's view

A man dressed suspiciously like an undertaker strolls up to you and withdraws a long string from a small bag he carries. He slowly starts taking the measurements of your body and with a satisfied nod packs his string away and strolls off once more.

Back to shop

The coffin maker walks back into the store, placing his small bag on the counter.

"No problem. Coffin'll be ready when he is."

Maybe if that person does actually kill the target, the coffin maker drags a coffin to where he died and does something...buries it, carts it off, what not.