Well, you've read more about this era than I did, but didn't Septimius Severus as one of his earliest actions completely overhaul the Praetorian Guard (i.e. essentially dismiss all that were in it and get only new guys in)? Which would argue that they had played some key role in making some or at least one of the immediately preceding Emperors.

BTW, Mike Duncan did say the auction didn't literally take place in the Senate but that some Praetorians essentially ran between candidates (who sensibly had barricaded themselves in their houses), telling them about the latest offer from the competition.

BTW, I don't remember, did we ever post my write-up of Fritz' editon of Montesquieu's Roman history at Rheinsberg?