Invest. First, press flag, as it breaks if you click on things before doing so. Then, press on the stock you want to buy shares of. Press "B" to buy shares and "S" to sell shares. If the stock price is negative, buying and selling will both remove money. If your money is negative, you go In Debt™. Each stock is unique.
I stole the idea and a lot of the code from @nose0224. According to Wikipedia, "In 12th-century France, the courtiers de change were concerned with managing and regulating the debts of agricultural communities on behalf of the banks. Because these men also traded with debts, they could be called the first [stock] brokers." As per the name "scuffed," the whole thing is abhorrently optimized and contains like 20 variables. I couldn't be bothered to leave any code comments. I reaaaally tried to get the noise from Scuffed Slots to play when you go In Debt™, but couldn't without breaking things. If you try to remix this and change stock names or characteristics, good luck.