When is it ok to buy houses? How many house should be purchased and how much money should be kept in reserve? When is it ok to mortgage a property or sell a house to buy another property?
When is it ok make a trade for a higher income property that involves increasing the chance that they will gain a monopoly, and vice versa? How much cash is it worth to buy/sell something that gives you/them a monopoly? What about trades that involved multiple properties? Is it ok to give them two lower income monopolies in exchange for a higher income one?
But, Monopoly is almost completely solved you don't really get to make many choices as the game is heavily influenced the RNG. And at this point the statisticians know exactly what choices to make.