Didius Julianus bought Rome. When Pertinax, the successor to Emperor Commodus, failed to support the Praetorian Guard in 193, they overthrew him and auctioned off the whole empire. Cassius Dio writes:

Didius Julianus, at once an insatiate money-getter and a wanton spendthrift, who was always eager for revolution and hence had been exiled by Commodus to his native city of Mediolanum, now, when he heard of the death of Pertinax, hastily made his way to the camp, and, standing at the gates of the enclosure, made bids to the soldiers for the rule over the Romans. Then ensued a most disgraceful business and one unworthy of Rome. For, just as if it had been in some market or auction-room, both the City and its entire empire were auctioned off. The sellers were the ones who had slain their emperor, and the would-be buyers were Sulpicianus and Julianus, who vied to outbid each other, one from the inside, the other from the outside. They gradually raised their bids up to twenty thousand sesterces per soldier. Some of the soldiers would carry word to Julianus, ‘Sulpicianus offers so much; how much more do you make it?’ And to Sulpicianus in turn, ‘Julianus promises so much; how much do you raise him?’ Sulpicianus would have won the day, being inside and being prefect of the city and also the first to name the figure twenty thousand, had not Julianus raised his bid no longer by a small amount but by five thousand at one time, both shouting it in a loud voice and also indicating the amount with his fingers. So the soldiers, captivated by this excessive bid and at the same time fearing that Sulpicianus might avenge Pertinax (an idea that Julianus put into their heads), received Julianus inside and declared him emperor.

He didn’t last long — Septimius Severus swept into the capital after nine weeks, and the Praetorians switched allegiance again. “And so it came about that Julianus was slain as he was reclining in the palace itself; his only words were, ‘But what evil have I done? Whom have I killed?’ He had lived sixty years, four months, and the same number of days, out of which he had reigned sixty-six days.”