I can't honestly take credit for this trick as it was Jessica's AI that first showed it to me, but it is a very handy method of making quick cash and manipulating the stock market in your favor. Works very nicely for the first few levels to keep cash in your pocket for buying more shops!
All you need is a vacant plot to make it work. Doesn't matter if the other shops in the district are unowned or owned by you or even other players. You buy stock in the District containing your vacant plot and then as soon as your next turn you can use this trick. Before you roll, renovate your vacant plot into a 3 star shop and the stock price will rise significantly. Then sell the stock off at the increased price, making a significant profit. Then renovate your 3-star shop back to one of the 200g options, which will give you 750g back for the sale of the 3-star shop. The stock price will return to it's original low price so you can wash, rinse and REPEAT as needed!
A few notes: I prefer to use balloon ports and tax offices for this trick, though anything will work. Check Points and Circus Tents will be reset to starting price every time you flip them back so best to avoid using them if you intend to use the trick frequently.
Like any stock trading, you want to buy in as low as possible for the highest profit. I also use this trick when trying to deal with freeloaders. I can flip the vacant plot to a 3-star (if I can afford it) to raise the stock price when I know a likely freeloader is about to visit the bank for salary/buying stocks and then flip it back to a Tax Office when I'm about to reach the bank so I can buy in at a lower rate.
Also, this trick works for acquiring the Stock Diving trophy. When I've raised a district up as much as I plan to, I can liquidate the stock and then flip the vacant plot back to a Tax Office and undercut any freeloaders who had climbed on.
Do be cautious about liquidating stock if you do plan to dominate and expand the district. You don't want to give freeloaders the advantage of having more stock in your district than you do!
And finally, don't worry if you can't land on a vacant plot. The AI really undervalues the vacant plots unless they already have 3 or so shops in the district already. I usually just use the buy option just as soon as anyone buys a vacant plot. I've yet to pay more than 3x value and the profit I can make off it easily makes up for it.
Hope that helps someone! Happy Insider Trading! --- ~The mystery of life isn't a problem to solve, but a reality to experience.~
Though, a few additional tips: after creating the three star shop and selling off your stock, don't change it back right away. You can keep it as a three star shop to prevent buyouts, and change it back whenever you want (like right before you cross the bank).
You can also do the capital trick, which requires a tiny bit of luck. Once you make the three star shop, try and land on yourself and then invest max into the shop, then sell all your stock then change it back. You get back all the money you invested along with the stock increase.