Self imposed restrictions can go a long way. The sim is homeless? Then they probably will not get hired for many jobs and may not have a selling license. Maybe they don't even have a legal ID. Your sim painted a valuable painting? But they are a nobody, so who would buy this? Ten lilies picked last week? Picked in whose garden, may I ask? Hello, police...
In the end no matter how creatively you come up with restrictions, this can only delay the problem, not remove it.
Alternately you could play scenarios where money doesn't play a role at all. My sims go adventuring a lot at the moment, they have 4 million simoleons, but also trust and love issues, so them being rich doesn't matter. I'm involved with their emotions, that is challenge enough.