Capitalism has a bunch of negative feedback loops - usually by becoming a problem you help solve it. If people notice that a particular good is about to become really expensive and switch sides to become a producer, they increase the supply of it and drive the price down. Even if they hoard and don't produce anything, they drive the price up early, when existing producers might still be able to increase production, and then drive the price down during the shortage as they unload their hoard. If people notice that renters are going to get fucked by rising rents and buy property to become landlords, they increase competition in the landlord market and drive rents down during the crisis.

The average person gets the initial capital to enter the game of capitalism by living like a poor person and banking the difference. That's how it works: this "capital" that earns a return is simply the difference between what people earn and what they consume. If you get a positive return on it you might eventually start being able to earn like a rich person and live like an average person.