I always befriended some people living alone and then moved them in so I can get their founds. I kept their furniture to replace some of my own and sold the rest. Then I fed them with a cowplant so I can get life potions to cheer up my sims when needed.
This is how my sim became a billionaire?
Other than murdering the neighbourhood, I collected things, used the money trash can, wrote a tons of books and painted a bit anf did some gardening. And hacked the bank through the computer.