Moreover, observing how extremely subject the city was to fire and falling down of house, by reason of their height and their standing so near together, he bought slaves that were builders and architects, and when he had collected these to the number of more than five hundred, he made it his practice to buy houses that were on fire, and those in the neighbourhood, which, in the immediate danger and uncertainty the proprietors were willing to part with for little or nothing, so that the greatest part of Rome, at one time or other, came into his hands.
tl;dr:
Crassus bought about 500 slaves that had experience in building houses and buildings, and when a property in Rome would catch fire or collapse, he would rush to the scene and convince the owners to sell the ruined property to him at a severely decreased price. At this point, he would salvage what he could, and use his slaves to rebuild and improve the property. He did this until he owned large swathes of the city and was one of the wealthiest individuals alive. (He had other schemes that were successful, but this is one of the big ones he’s known for.)
Source:
Plutarch, John Dryden, and Arthur Hugh Clough. "Crassus." Plutarch's Lives. New York: Modern Library, 2001. 725. Print.
Further Reading:
Lol, I remember reading this in Plutarch's Lives, it's just one of those passages that reminds you that people have been pulling the same scams throughout human history.