I'm currently reading Robert Harris's Cicero trilogy (fascinating) and you cand find in it the precise way he used to buy up all properties. At this time Rome didn't have proper firemen squads, so Crassus created his own. When a building was in flame, his team turned up and would put out the fire only as soon as the people would agree to sell the property to Crassus (for a fraction of the real price of course). He was a clever bastard.