He was helping the rich hide their money, sometimes other people's money too.

The BBC documentary on the Maxwells was very interesting on this. Most people try to sell the version of the story that Ghislaine Maxwell met Epstein after her father died and got taken in by him because she was looking for a replacement father figure. However she was actually meeting Epstein while her father was still alive because he was helping them move their money off shore (this included money they stole from the company pension scheme).

So there's 3 streams of income we can guess at. Ponzi scheme, tax evasion schemes and blackmail.