If you have the highschool pack, you can sell outfits for crazy amounts of simoleons. Dragon fruit is expensive, also you could try getting a money tree. If you have horse pack, aged bottles of nectar sell for 2000 a bottle. You could try buying paintings from community spaces or art academy and then selling them on plopsy or buy from the flea market and then sell again with max prices. As last resort, searching for stuff in the dumpster usually gives expensive appliances and furniture you can fix and sell if you have the eco pack. The ray gun from science career can change items from a cheap one to a more expensive one, I forgot what the interaction is actually called but that's how one of my sims was able change her fridge. You could use that to change stuff and then sell it.