What type of business did Scrooge have?

Scrooge and Marley (deceased in the story) were moneylenders, with Bob Cratchit as the clerk.

Have to get the money from somewhere. according to the book he was broke so he could not marry Bell.

He started with very little. From a sign on his building it said "Importer of fine goods and china." Many a wealthy started as importers back then. So he was an importer and then got into money loaning. He probably partnered with Marley as an importer (Marley probably have connections to get started), but loaning money was less risky and with higher returns and they were getting to old for traveling. He no doubt got into real estate because of his prepossessing homes, when people did not pay the debt.

He was a cheap so he let his home go. That is why it looked so bad.