(1) Dumpster diving is the best. If you choose freegan trait it ups your chances for getting high cost things. Sell them straight from your inventory; don't put them on your lot. Even if it's junk you'll get full price that way.
(2) Frogs are good. Llamas, cows, and chickens are good. They don't require skills. If you set up your sim with a nature aspiration that will increase the quality of what they collect and make it easier to complete the Collector aspiration, which will give you a lot of aspiration points.
(3) There are a lot of aspirations where you don't need skills for the first stage. Complete as many of those as you can, and fulfill as many wishes as you can.
(4) Then get a money tree.
(4) If you keep going with the chickens, eventually, with luck, you'll get a golden egg, or can buy a golden treat to feed one of your chickens. Make sure the golden chicken is near your money tree, so it will become perfect. A perfect money tree will net you $31,000 a day.
If you don't mind cheating, once you have the llamas/cows you can cheat to get animals clothes to sell.
Also you can buy the attic stack and keep rummaging for decorations and sell them.