I find the self-employed fisherman one of the easiest self-employed careers to level up. They can usually catch 6 Deathfish a night at the graveyard, easily netting $30,000 or so at the Consignment Shop. That's more than most of my sculptors can make in a full day of working, and it's more than most of my painters have made too. Somewhere here there's a thread in which a player posted about making way more money than that with their Deathfish; I don't remember the numbers, but they were huge.