Marley is a physical and metaphorical representation for how if making money and materialism is the main purpose to your life, then it will be the main reason for your eternal suffering
Marley is symbolic of the eternal consequences of the pursuit of materialism- in his life on earth he shunned poverty, indulging in his avarice (greed), thus in his afterlife he experiences spiritual poverty.
Dickens utilises Marley as his mouthpiece to be scathing of the pervasive social ills of society and to challenge the upper class notion that materialism can replace genuine community.
Dickens' use of gothic elements
Supernatural
Morality intertwined with supernatural, uncanny and realm of endless possibilities