The sign that hung above the door still read Scrooge and Marley, though Marley had been dead for 7 years.

Marley was dead to begin with. There is no doubt about that. His burial  was signed by the clergyman  and by Scrooge. Old Marley was as dead as a door-nail . You must know that or nothing else seems magical . Scrooge was Marley's business partner in life.