Some of Crassus' wealth was acquired conventionally, through traffic in slaves, production from silver mines, and speculative real estate purchases. Crassus tended to specialize in deals involving proscribed citizens and especially and notoriously purchasing during fires or structural collapse of buildings. When buildings were burning, Crassus and his purposely-trained crew would show up, and Crassus would offer to purchase the presumably doomed property and perhaps neighboring endangered properties from their owners for speculatively low sums; if the purchase offer was accepted, Crassus would then use his army of some 500 slaves which he purchased due to their knowledge of architecture and building to put the fire out, sometimes before too much damage had been done: otherwise Crassus would use his crews to rebuild. If his purchase offers were not accepted, then Crassus would not engage in firefighting. Crassus's slaves employed the Roman method of firefighting—destroying the burning building to curtail the spread of the flames. Similar methods were used by Crassus in the common event of the collapse of the large Roman buildings known as insulae, which were notorious for their poor construction and unsafe conditions. Crassus was happy to cheaply construct new insulae using his slave labour force, in place of the old insulae which had collapsed and/or burned; however, he was known for his raising of rents rather than for his erection of improved residential structures.
The bolded bits are the relevant part, the bit that blew my mind when I heard about it. (I mean, seriously, what a jerk.)
*Apparently at the time of his death, he was worth 7100 talents; a talent was a fortune, and that total is apparently roughly equivalent to nearly eight and a half billion dollars.
It's also interesting to note that Crassus was a major backer of Julius Caesar, and was also one of the three men (along with Caesar) to become the first triumvirate.