I'm at 100 million so far selling real estate.
I sell identical houses filled to the brim with fish tanks, worth 4200000, and I get about 500 000 per sale.
This is the best way I've found to make money, but it takes ages getting that far.
What are your ways, and what do you do?

As a real estate professional IRL, I like the idea of house flipping, but if I'm understanding you correctly, you're getting these fish tanks at wholesale prices and filling the houses with an unrealistic amount of fish tanks, literally filling all of the tiles with them, and then selling the home. If I'm not mistaken, the fish tanks, save for a few of them, would serve no purpose to a future homeowner (and it seems unlikely that anyone would want to buy a house like that, with the fish tanks included in the deal, Sim or otherwise). To me, it seems motherlode would be a lot easier cheat.

Now if you were selling warehouses like this...

Made? Probably like 200,000 from selling multiple houses

I don't bother. My wealthier sims tend to buy and sell real estate or cars, but they also tend to buy up new business, holiday houses etc as soon as they have it. Most of my sims are poor and scraping by and that's how I like it. If I wanted an obscenely rich sim I would simply motherlode them the money. I would find selling houses full of fish tanks to be tedious.

Venue business, filled with attractive objects (hot tubs, poker tables, instruments, whatever). Sim with a gold badge in sales. Put the price up disgustingly high, dazzle customers into paying. If you have snapdragons, surround the fun items with them to make customers stay longer. If your salesperson isn't a servo or plant sim, use espresso or the energizer or date boosts to keep them going. Then watch the money roll in. If you've got plenty of snapdragons on the lot, you can make your sim meditate once the venue is filled with customers, put the game on speed three, and go around doing other things, occasionally checking in on the status of the game.

(Hey, I never said it was fun. But if you just want the most efficient way to make money in-game...)

Also, once you have millions in the account, give every available sim the fortune sim perk of investments, I forget what it's called, and the money will roll in without you even having to do anything. Yeah, sometimes they'll lose money, but that's nothing compared to what they'll make.

I have sims with 200000+, and is either because they had relatives who passed away of old age or promotions (reaching the top of their career).

Haha, technically they are community lots, so I suppose they could be warehouses! I don't consider it a cheat, because, I mean, it's not. To each his own, sims is a very broad game with a lot of things to do, and a lot of different playing styles. Maybe I'm greedy but I find collecting obscene amounts of money to be fun! Strictly no cheats! Building up a 10 star business, earning badges, socialising, etc. etc, I find it to be a lot more fun than just typing in a cheat code.

It's more of a strategic way to play the game, to see how the in-game features can be exploited to the max. That can be pretty fun. But mostly I like to play according to a kind of logic, so I agree that I wouldn't sell a house which wasn't functional. But perhaps something like a real estate business which specialised in different warehouses (to be used for a sim's business in the future) or houses filled with precious furniture and art, I could see.

One of my houses is a kind of spiritual retreat, with a Grand Master and five disciples. Every Sunday night the disciples select one of their number to leave (votes based on lifetime relationship scores) and join my main community. On the Monday night, his or her replacement arrives with the standard 20k of funds. Grand Masters are likewise replaced when they die, as well as disciples who die of old age in the lodge. I've been playing this house for over 130 sim-days now and it's worth well over 750,000, even with departees leaving with 10% of lodge funds as goodies in their inventory. (The next one to go will take eleven Gray Woman statues with him/her.)

Malcolm Landgraab IV made $30,000 today alone. I was pretty proud of him. Club Dante is really starting to hit off, plus that extra money is going to be used to make a nursery for his first child who will be born next rotation, with enough money left over to buy another nightclub.

Mine are poor. Even the rich ones.

Uhhhh I don't know I think I used the motherlode cheat to make my Sims millionaires. But not cheating I think I got a little over 50,000.

The most money I ever made was with a sim who joined a custom career I downloaded that had a faulty chance card. Instead of being awarded $3000, or $30,000 whichever it was supposed to be, it awarded $3,000,000. Needless to say that sim never had to work another day in her life, nor did her children....

I think 50.000 through a chance card and I wasn't too happy about that.
Don't really like it if my sims have a lot of money or gain it that quickly..

When we were young my brother and I would do a competition, we both had our own household (I used the Singles one for it, but he made one himself) and we each tried to earn the most money with those Sims in the shortest amount of time. I honestly can't remember how much we had earned when we stopped though... or who won, for that matter!

In my Pleasantview reboot, a young Beau Broke moves out of home with $50, half a pizza and a few coupons. Oh and his childhood bed.

He was so poor (no free money mod in play), he had to crash that first night on the hermit's couch, deep in the woods. Then he moves into his girlfriend's trailer park shack . Together they are just scraping by.

The third day Beau goes off to work. He's a Slacker. He finds himself in a recoding studio and comes home, all dazed and confused, with $80,000 in his pocket.

Pretty good for a 3 day old adult.