Anyone who plans to use the stock market to get a massive amount of money should be cautious that the game uses a signed 32-Bit integer (i.e any value higher then 2,147,483,647 will roll over into a negative value)
This is actually very easy to do on a replay or anyone who holds out the Assassination missions.
At the end game I had about 36 Million split between all three characters. Redwood Juror Assassination left me with 280 Milllion (Invest in Debonaire before Mission, Do Mission, Wait until Debonaire stocks raise, sell off Debonaire shares, invest all in Redwood, wait until Redwood stabilizes, sell all of Redwood stocks.
After the iFruit Assassination I had about 560 Million, and the Vapid Assassination it jumped to 980 Million. Gold Coast bumped me up to 1.6 Billion.
This is the point where everyone should generally stop. I happened to get that random hitchhiker mission after the story mode and my stock market stuff, telling me the hint about Tinkle Stocks, I decided to dump my 1.6 Billion into that. I figured why not.
smashed up my car and tried to fix it I was surprised to see the game told me I could not afford to repair it and noticed my money was in a negative value.
Have fun with the stock market but don't go too crazy.